In the context of whether or not nearby stars were created from the same nebula, this answer states:
imagine two stars with very similar orbits, one with a period of 200 million years and the other with a period of 210 million years. If they start off right next to each other, then after 2 billion years, the first star has made 10 complete orbits, while the second has made about 9.5 -- meaning it will now be on the other side of the galaxy from the first star.
How close would the two stars have originally been to have such different orbital periods?
Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae  and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.
Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them.
Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure,  extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe, with only about 10% containing bars about 8 billion years ago, to roughly a quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe (Hubble volume) have bars. 
The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc.  The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope. 
Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe.  They are mostly found in low-density regions and are rare in the centers of galaxy clusters. 
28.2 Galaxy Mergers and Active Galactic Nuclei
One of the conclusions astronomers have reached from studying distant galaxies is that collisions and mergers of whole galaxies play a crucial role in determining how galaxies acquired the shapes and sizes we see today. Only a few of the nearby galaxies are currently involved in collisions, but detailed studies of those tell us what to look for when we seek evidence of mergers in very distant and very faint galaxies. These in turn give us important clues about the different evolutionary paths galaxies have taken over cosmic time. Let’s examine in more detail what happens when two galaxies collide.
Mergers and Cannibalism
Figure 28.1 shows a dynamic view of two galaxies that are colliding. The stars themselves in this pair of galaxies will not be affected much by this cataclysmic event. (See the Astronomy Basics feature box Why Galaxies Collide but Stars Rarely Do.) Since there is a lot of space between the stars, a direct collision between two stars is very unlikely. However, the orbits of many of the stars will be changed as the two galaxies move through each other, and the change in orbits can totally alter the appearance of the interacting galaxies. A gallery of interesting colliding galaxies is shown in Figure 28.7. Great rings, huge tendrils of stars and gas, and other complex structures can form in such cosmic collisions. Indeed, these strange shapes are the signposts that astronomers use to identify colliding galaxies.
Why Galaxies Collide but Stars Rarely Do
Throughout this book we have emphasized the large distances between objects in space. You might therefore have been surprised to hear about collisions between galaxies. Yet (except at the very cores of galaxies) we have not worried at all about stars inside a galaxy colliding with each other. Let’s see why there is a difference.
The reason is that stars are pitifully small compared to the distances between them. Let’s use our Sun as an example. The Sun is about 1.4 million kilometers wide, but is separated from the closest other star by about 4 light-years, or about 38 trillion kilometers. In other words, the Sun is 27 million of its own diameters from its nearest neighbor. If the Sun were a grapefruit in New York City, the nearest star would be another grapefruit in San Francisco. This is typical of stars that are not in the nuclear bulge of a galaxy or inside star clusters. Let’s contrast this with the separation of galaxies.
The visible disk of the Milky Way is about 100,000 light-years in diameter. We have three satellite galaxies that are just one or two Milky Way diameters away from us (and will probably someday collide with us). The closest major spiral is the Andromeda Galaxy (M31), about 2.4 million light-years away. If the Milky Way were a pancake at one end of a big breakfast table, M31 would be another pancake at the other end of the same table. Our nearest large galaxy neighbor is only 24 of our Galaxy’s diameters from us, and it will begin to crash into the Milky Way in about 4.5 billion years.
Galaxies in rich clusters are even closer together than those in our neighborhood (see The Distribution of Galaxies in Space). Thus, the chances of galaxies colliding are far greater than the chances of stars in the disk of a galaxy colliding. And we should note that the difference between the separation of galaxies and stars also means that when galaxies do collide, their stars almost always pass right by each other like smoke passing through a screen door.
The details of galaxy collisions are complex, and the process can take hundreds of millions of years. Thus, collisions are best simulated on a computer (Figure 28.8), where astronomers can calculate the slow interactions of stars, and clouds of gas and dust, via gravity. These calculations show that if the collision is slow, the colliding galaxies may coalesce to form a single galaxy.
When two galaxies of equal size are involved in a collision, we call such an interaction a merger (the term applied in the business world to two equal companies that join forces). But small galaxies can also be swallowed by larger ones—a process astronomers have called, with some relish, galactic cannibalism (Figure 28.9).
The very large elliptical galaxies we discussed in Galaxies probably form by cannibalizing a variety of smaller galaxies in their clusters. These “monster” galaxies frequently possess more than one nucleus and have probably acquired their unusually high luminosities by swallowing nearby galaxies. The multiple nuclei are the remnants of their victims (Figure 28.9). Many of the large, peculiar galaxies that we observe also owe their chaotic shapes to past interactions. Slow collisions and mergers can even transform two or more spiral galaxies into a single elliptical galaxy.
A change in shape is not all that happens when galaxies collide. If either galaxy contains interstellar matter, the collision can compress the gas and trigger an increase in the rate at which stars are being formed—by as much as a factor of 100. Astronomers call this abrupt increase in the number of stars being formed a starburst , and the galaxies in which the increase occurs are termed starburst galaxies (Figure 28.10). In some interacting galaxies, star formation is so intense that all the available gas is exhausted in only a few million years the burst of star formation is clearly only a temporary phenomenon. While a starburst is going on, however, the galaxy where it is taking place becomes much brighter and much easier to detect at large distances.
When astronomers finally had the tools to examine a significant number of galaxies that emitted their light 11 to 12 billion years ago, they found that these very young galaxies often resemble nearby starburst galaxies that are involved in mergers: they also have multiple nuclei and peculiar shapes, they are usually clumpier than normal galaxies today, with multiple intense knots and lumps of bright starlight, and they have higher rates of star formation than isolated galaxies. They also contain lots of blue, young, type O and B stars, as do nearby merging galaxies.
Galaxy mergers in today’s universe are rare. Only about five percent of nearby galaxies are currently involved in interactions. Interactions were much more common billions of years ago (Figure 28.11) and helped build up the “more mature” galaxies we see in our time. Clearly, interactions of galaxies have played a crucial role in their evolution.
Active Galactic Nuclei and Galaxy Evolution
While galaxy mergers are huge, splashy events that completely reshape entire galaxies on scales of hundreds of thousands of light-years and can spark massive bursts of star formation, accreting black holes inside galaxies can also disturb and alter the evolution of their host galaxies. You learned in Active Galaxies, Quasars, and Supermassive Black Holes about a family of objects known as active galactic nuclei (AGN), all of them powered by supermassive black holes. If the black hole is surrounded by enough gas, some of the gas can fall into the black hole, getting swept up on the way into an accretion disk, a compact, swirling maelstrom perhaps only 100 AU across (the size of our solar system).
Within the disk the gas heats up until it shines brilliantly even in X-rays, often outshining the rest of the host galaxy with its billions of stars. Supermassive black holes and their accretion disks can be violent and powerful places, with some material getting sucked into the black hole but even more getting shot out along huge jets perpendicular to the disk. These powerful jets can extend far outside the starry edge of the galaxy.
AGN were much more common in the early universe, in part because frequent mergers provided a fresh gas supply for the black hole accretion disks. Examples of AGN in the nearby universe today include the one in galaxy M87 (see Figure 27.7), which sports a jet of material shooting out from its nucleus at speeds close to the speed of light, and the one in the bright galaxy NGC 5128, also known as Centaurus A (see Figure 28.12).
Many highly accelerated particles move with the jets in such galaxies. Along the way, the particles in the jets can plow into gas clouds in the interstellar medium, breaking them apart and scattering them. Since denser clouds of gas and dust are required for material to clump together to make stars, the disruption of the clouds can halt star formation in the host galaxy or cut it off before it even begins.
In this way, quasars and other kinds of AGN can play a crucial role in the evolution of their galaxies. For example, there is growing evidence that the merger of two gas-rich galaxies not only produces a huge burst of star formation, but also triggers AGN activity in the core of the new galaxy. That activity, in turn, could then slow down or shut off the burst of star formation—which could have significant implications for the apparent shape, brightness, chemical content, and stellar components of the entire galaxy. Astronomers refer to that process as AGN feedback, and it is apparently an important factor in the evolution of most galaxies.
Distance & Speed Of Sun’s Orbit Around Galactic Centre Measured
In 2013, the European Space Agency deployed the long-awaited Gaia space observatory. As one of a handful of next-generation space observatories that will be going up before the end of the decade, this mission has spent the past few years cataloging over a billion astronomical objects. Using this data, astronomers and astrophysicists hope to create the largest and most precise 3D map of the Milky Way to date.
Though it is almost to the end of its mission, much of its earliest information is still bearing fruit. For example, using the mission’s initial data release, a team of astrophysicists from the University of Toronto managed to calculate the speed at which the Sun orbits the Milky Way. From this, they were able to obtain a precise distance estimate between our Sun and the center of the galaxy for the first time.
For some time, astronomers have been unsure as to exactly how far our Solar System is from the center of our galaxy. Much of this has to do with the fact that it is impossible to view it directly, due to a combination of factors (i.e. perspective, the size of our galaxy, and visibility barriers). As a result, since the year 2000, official estimates have varied between 7.2 and 8.8 kiloparsecs (
23,483 to 28,700 light years).
Infrared image from Spitzer Space Telescope, showing the stars at the center of the Milky Way Galaxy. Credit: NASA/JPL-Caltech/S. Stolovy (SSC/Caltech)
For the sake of their study, the team – which was led by Jason Hunt, a Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics at the University of Toronto – combined Gaia’s initial release with data from the RAdial Velocity Experiment (RAVE). This survey, which was conducted between 2003 and 2013 by the Australian Astronomical Observatory (AAO), measured the positions, distances, radial velocities and spectra of 500,000 stars.
Over 200,000 of these stars were also observed by Gaia and information on them was included in its initial data release. As they explain in their study, which was published in the Journal of Astrophysical Letters in November 2016, they used this to examined the speeds at which these stars orbit the center of the galaxy (relative to the Sun), and in the process discovered that there was an apparent distribution in their relative velocities.
In short, our Sun moves around the center of the Milky Way at a speed of 240 km/s (149 mi/s), or 864,000 km/h (536,865 mph). Naturally, some of the more than 200,000 candidates were moving faster or slower. But for some, there was no apparent angular momentum, which they attributed to these stars being scattering onto “chaotic, halo-type orbits when they pass through the Galactic nucleus”.
As Hunt explained in Dunlap Institute press release:
Detection of an unusually bright X-Ray flare from Sagittarius A*, a supermassive black hole in the center of the Milky Way galaxy. Credit: NASA/CXC/Stanford/I. Zhuravleva et al.
“Stars with very close to zero angular momentum would have plunged towards the Galactic center where they would be strongly affected by the extreme gravitational forces present there. This would scatter them into chaotic orbits taking them far above the Galactic plane and away from the Solar neighbourhood… By measuring the velocity with which nearby stars rotate around our Galaxy with respect to the Sun, we can observe a lack of stars with a specific negative relative velocity. And because we know this dip corresponds to 0 km/sec, it tells us, in turn, how fast we are moving.”
The next step was to combine this information with proper motion calculations of Sagittarius A* – the supermassive black hole believed to be at the center of our galaxy. After correcting for its motion relative to background objects, they were able to effectively triangulate the Earth’s distance from the center of the galaxy. From this, they derived a refined distance of estimate of 7.6 to 8.2 kpc – which works out to about 24,788 to 26,745 light years.
This study builds upon previous work conducted by the study’s co-authors – Prof. Ray Calberg, the current chair of the Department of Astronomy & Astrophysics at the University of Toronto. Years ago, he and Prof. Kimmo Innanen of the Department of Physics and Astronomy at York University conducted a similar study using radial velocity measurement from 400 of the Milky Way’s stars.
But by incorporating data from the Gaia observatory, the UofT team was able to obtain a much more comprehensive data set and narrow the distance to galactic center by a significant amount. And this was based on only the initial data released by the Gaia mission. Looking ahead, Hunt anticipates that further data releases will allow his team and other astronomers to refine their calculations even more.
“Gaia’s final release in late 2017 should enable us to increase the precision of our measurement of the Sun’s velocity to within approximately one km/sec,” he said, “which in turn will significantly increase the accuracy of our measurement of our distance from the Galactic center.”
As more next-generation space telescopes and observatories are deployed, we can expect them to provide us with a wealth of new information about our Universe. And from this, we can expect that astronomers and astrophysicists will begin to shine the light on a number of unresolved cosmological questions.
The designation S0–2 was first used in 1998. S0 indicates a star within one arc-second of Sgr A*, indicating the galactic centre, and S0–2 was the second closest star seen at the time of the measurements.  The star had been catalogued simply as S2 a year earlier, the second of eleven infrared sources near the galactic centre, numbered approximately anti-clockwise.  It is a coincidence that the star is numbered "2" in both lists other catalogs number it differently. 
The highly eccentric orbit of S2 will give astronomers an opportunity to test for various effects predicted by general relativity and even extra-dimensional effects.  These effects reached a maximum at closest approach, which occurred in mid-2018.   Given a recent estimate of 4.31 million M ☉ for the mass of the Sgr A* black hole and S2's close approach, this makes S2 the fastest known ballistic orbit, reaching speeds exceeding 5,000 km/s (11,000,000 mph, or 1 ⁄ 60 the speed of light) and acceleration of about 1.5 m/s 2 (almost one-sixth of Earth's surface gravity). 
The motion of S2 is also useful for detecting the presence of other objects near to Sgr A*. It is believed that there are thousands of stars, as well as dark stellar remnants (stellar black holes, neutron stars, white dwarfs) distributed in the volume through which S2 moves. These objects will perturb S2's orbit, causing it to deviate gradually from the Keplerian ellipse that characterizes motion around a single point mass.  So far, the strongest constraint that can be placed on these remnants is that their total mass comprises less than one percent of the mass of the supermassive black hole. 
2018 pericentre passage Edit
In 2018, S2 made its closest approach to Sgr A*, reaching 7650 km/s or almost 3% of the speed of light, while passing the black hole at a distance of just 120 AU or about 1400 times its Schwarzschild radius.   S2 reached its pericenter on May 19, 2018, while its velocity in the line of sight from Earth peaked in April, and later hit its minimum in late August and early September. 
Independent analyses by the GRAVITY collaboration     (led by Reinhard Genzel) and the KECK/UCLA Galactic Center Group   (led by Andrea Ghez) revealed a combined transverse Doppler and gravitational redshift up to 200 km/s/c, in agreement with general relativity predictions.
Additional analyis has revealed a Schwarzschild precession of 12 arcminutes (0.2 degrees) in S2's orbit caused by the close passage, fully consistent with general relativity.   
In 2012, a star called S0–102 (or S55) was found to be orbiting even closer to the Milky Way's central supermassive black hole than S0–2 does. At one-sixteenth the brightness of S0–2, S0–102 was not initially recognized because it required many more years of observations to distinguish it from its local infrared background. S0–102 has an orbital period of 11.5 years, even shorter than that of S0–2. Of all the stars orbiting the black hole, only these two have their orbital parameters and trajectories fully known in all three dimensions of space.  The discovery of two stars orbiting the central black hole so closely with their orbits fully described is of extreme interest to astronomers, as the pair together will allow much more precise measurements on the nature of gravity and general relativity around the black hole than would be possible from using S0–2 alone. [ citation needed ]
A still closer star S62 has since been discovered with an orbital period of 9.9 years.
The Nearest Neighbor Star
The image on the preceding page was created to demonstrate that Alpha Centauri is not a star, but really a star system. Of the three stars in the system, the dimmest - called Proxima Centauri - is actually the nearest star to the Sun. The two bright stars, called Alpha Centauri A and B form a close binary system they are separated by only 23 times the Earth - Sun distance. This is slightly greater than the distance between Uranus and the Sun.
The Alpha Centauri system is not visible from much of the northern hemisphere. The below image shows this star system and other objects near it in the sky.
Proxima Centauri, the closest star to our own, is still 40,208,000,000,000 km away. (Or about 268,770 AU.) When we talk about the distances to the stars, we no longer use the AU, or Astronomical Unit commonly, the light year is used. A light year is the distance light travels in one year - it is equal to 9.461 x 10 12 km. Alpha Centauri A & B are roughly 4.35 light years away from us. Proxima Centauri is slightly closer at 4.25 light years.
How Do We Calculate Distances of This Magnitude?
The methods astronomers use to measure distances to the stars are pieces of fundamental and active work in astronomy with important implications for how we understand the Universe around us.
One of the most accurate methods astronomers use to measure distances to stars is called parallax. If you hold your finger in front of your face and close one eye and look with the other, then switch eyes, you'll see your finger seem to "shift " with respect to more distant objects behind it. This is because your eyes are separated from each other by a few inches - so each eye sees the finger in front of you from a slightly different angle. The amount your finger seems to shift is called its "parallax".
Astronomers can measure parallax by measuring the position of a nearby star very carefully with respect to more distant stars behind it, then measuring those positions again six months later when the Earth is on the opposite side of its orbit. If the star is close enough to us, a measurable parallax will be seen: the position of the star relative to the more distant background stars will have shifted. The shift is tiny - less than an arcsecond even for the nearest star. (An arcsecond is 1/60 of an arcminute, which is 1/60 of a degree.) (Imagine the Universe has more information on calculating parallax.)
Why Are These Distances Important To Astronomers?
Stars are not actually stationary objects! The Galaxy is rotating, and the stars are in orbit around its center. Not every star moves at the same rate - how fast they orbit can depend on where the star is located within the Galaxy. Our Sun, being fairly far from the Galactic Center, takes over 200 million years to circle the Galaxy once. Some of the stars near us are moving faster than us, and some slower. As Phil Plaitt, from Bad Astronomy says, ". like cars on a highway, stars continually pass each other as they orbit the Galaxy. They change positions, slowly, but measurably."
This animation by Frog Rock Observatory shows the movement of Barnard's Star across the sky from 1985 to 2005. Barnard's Star is approaching the Sun so rapidly that around 11,700 AD, it will be 3.8 light years from the Sun - and thus the closest star to our own! (Garcia-Sanchez, et al, 2001)
The Voyager 1 spacecraft is on an interstellar mission. It is traveling away from the Sun at a rate of 17.3 km/s. If Voyager were to travel to Proxima Centauri, at this rate, it would take over 73,000 years to arrive. If we could travel at the speed of light, an impossibility due to Special Relativity, it would still take 4.22 years to arrive!
Why Can't We Travel Faster Than the Speed Of Light?
According to Special Relativity the mass of an object increases as its speed increases, and approaches infinity as the object's speed approaches the speed of light. This means that it would take an infinite amount of energy to accelerate an object to the speed of light.
There's no fundamental reason why we can't get as close to the speed of light as we like, provided we have enough energy. But this is probably far in the future.
Galactic orbits and distance between stars - Astronomy
Is it true that, as we follow the planets outward from the sun, the distances become about double each time? Does that mean that Venus is closer to Earth than Mars is?
Yes, it is true that there is somewhat of a pattern to the distances of the planets from the Sun. Venus is 1.8 times as far from the Sun as Mercury, and Earth is about 1.4 times as far from the sun as Venus. Mars is 1.5 times farther than Earth. This seems to be a pattern - each planet could be between 1.4 and 1.8 times farther from the sun than its "inside" neighbor. Then comes the problem - Jupiter is 3.4 times farther from the sun than Mars. This is where the pattern falls apart, although some say that the asteroid belt, which is in between Jupiter and Mars, could count as a substitute for a planet. Then Saturn is 1.8 times farther than Jupiter, Uranus is 2 times farther than Saturn, and Neptune is 1.6 times farther from the Sun than Uranus. Pluto doesn't fit this pattern at all. So there seems to be some sort of pattern to this, but there's no real theory that explains why the planets ended up at the distances they did, so it could also be a complete coincidence that they're somewhat evenly spaced.
So the "doubling" rule does work, but only approximately. This means that yes, the difference between the average orbital distance of Mars from the Sun to the average orbital distance of Earth from the Sun is greater (about 78 million km) than the difference between the Earth's average orbital distance from the Sun to Venus' average orbital distance from the Sun (41 million km). However, since the distance between the Earth and other planets depends not only on the size of their orbits but also on where they are in their orbits relative to each other, Venus is not always closer to Earth than Mars is.
This page was last updated on July 18, 2015.
About the Author
Cathy got her Bachelors degree from Cornell in May 2003 and her Masters of Education in May 2005. She did research studying the wind patterns on Jupiter while at Cornell. She is now an 8th grade Earth Sciences teacher in Natick, MA.
25.5 Stellar Populations in the Galaxy
In the first section of his chapter, we described the thin disk, thick disk, and stellar halo. Look back at Table 25.1 and note some of the patterns. Young stars lie in the thin disk, are rich in metals, and orbit the Galaxy’s center at high speed. The stars in the halo are old, have low abundances of elements heavier than hydrogen and helium, and have highly elliptical orbits randomly oriented in direction (see Figure 25.19). Halo stars can plunge through the disk and central bulge, but they spend most of their time far above or below the plane of the Galaxy. The stars in the thick disk are intermediate between these two extremes. Let’s first see why age and heavier-element abundance are correlated and then see what these correlations tell us about the origin of our Galaxy.
Two Kinds of Stars
The discovery that there are two different kinds of stars was first made by Walter Baade during World War II. As a German national, Baade was not allowed to do war research as many other U.S.-based scientists were doing, so he was able to make regular use of the Mount Wilson telescopes in southern California. His observations were aided by the darker skies that resulted from the wartime blackout of Los Angeles.
Among the things a large telescope and dark skies enabled Baade to examine carefully were other galaxies—neighbors of our Milky Way Galaxy. We will discuss other galaxies in the next chapter (Galaxies), but for now we will just mention that the nearest Galaxy that resembles our own (with a similar disk and spiral structure) is often called the Andromeda galaxy , after the constellation in which we find it.
Baade was impressed by the similarity of the mainly reddish stars in the Andromeda galaxy’s nuclear bulge to those in our Galaxy’s globular clusters and the halo. He also noted the difference in color between all these and the bluer stars found in the spiral arms near the Sun (Figure 25.20). On this basis, he called the bright blue stars in the spiral arms population I and all the stars in the halo and globular clusters population II .
We now know that the populations differ not only in their locations in the Galaxy, but also in their chemical composition, age, and orbital motions around the center of the Galaxy. Population I stars are found only in the disk and follow nearly circular orbits around the galactic center. Examples are bright supergiant stars, main-sequence stars of high luminosity (spectral classes O and B), which are concentrated in the spiral arms, and members of young open star clusters. Interstellar matter and molecular clouds are found in the same places as population I stars.
Population II stars show no correlation with the location of the spiral arms. These objects are found throughout the Galaxy. Some are in the disk, but many others follow eccentric elliptical orbits that carry them high above the galactic disk into the halo. Examples include stars surrounded by planetary nebulae and RR Lyrae variable stars. The stars in globular clusters, found almost entirely in the Galaxy’s halo, are also classified as population II.
Today, we know much more about stellar evolution than astronomers did in the 1940s, and we can determine the ages of stars. Population I includes stars with a wide range of ages. While some are as old as 10 billion years, others are still forming today. For example, the Sun, which is about 5 billion years old, is a population I star. But so are the massive young stars in the Orion Nebula that have formed in the last few million years. Population II, on the other hand, consists entirely of old stars that formed very early in the history of the Galaxy typical ages are 11 to 13 billion years.
We also now have good determinations of the compositions of stars. These are based on analyses of the stars’ detailed spectra. Nearly all stars appear to be composed mostly of hydrogen and helium, but their abundances of the heavier elements differ. In the Sun and other population I stars, the heavy elements (those heavier than hydrogen and helium) account for 1–4% of the total stellar mass. Population II stars in the outer galactic halo and in globular clusters have much lower abundances of the heavy elements—often less than one-hundredth the concentrations found in the Sun and in rare cases even lower. The oldest population II star discovered to date has less than one ten-millionth as much iron as the Sun, for example.
As we discussed in earlier chapters, heavy elements are created deep within the interiors of stars. They are added to the Galaxy’s reserves of raw material when stars die, and their material is recycled into new generations of stars. Thus, as time goes on, stars are born with larger and larger supplies of heavy elements. Population II stars formed when the abundance of elements heavier than hydrogen and helium was low. Population I stars formed later, after mass lost by dying members of the first generations of stars had seeded the interstellar medium with elements heavier than hydrogen and helium. Some are still forming now, when further generations have added to the supply of heavier elements available to new stars.
The Real World
With rare exceptions, we should never trust any theory that divides the world into just two categories. While they can provide a starting point for hypotheses and experiments, they are often oversimplifications that need refinement a research continue. The idea of two populations helped organize our initial thoughts about the Galaxy, but we now know it cannot explain everything we observe. Even the different structures of the Galaxy—disk, halo, central bulge—are not so cleanly separated in terms of their locations, ages, and the heavy element content of the stars within them.
The exact definition of the Galaxy’s disk depends on what objects we use to define it, and, as we saw earlier, it has no sharp boundary. The hottest young stars and their associated gas and dust clouds are mostly in a region about 200 light-years thick. Older stars define a thicker disk that is about 2000 light-years thick. Halo stars spend most of their time high above or below the disk but pass through it on their highly elliptical orbits and so are sometimes found relatively near the Sun.
The highest density of stars is found in the central bulge, that bar-shaped inner region of the Galaxy. There are a few hot, young stars in the bulge, but most of the bulge stars are more than 10 billion years old. Yet unlike the halo stars of similar age, the abundance of heavy elements in the bulge stars is about the same as in the Sun. Why would that be?
Astronomers think that star formation in the crowded nuclear bulge occurred very rapidly just after the Milky Way Galaxy formed. After a few million years, the first generation of massive and short-lived stars then expelled heavy elements in supernova explosions and thereby enriched subsequent generations of stars. Thus, even stars that formed in the bulge more than 10 billion years ago started with a good supply of heavy elements.
Exactly the opposite occurred in the Small Magellanic Cloud , a small galaxy near the Milky Way, visible from Earth’s Southern Hemisphere. Even the youngest stars in this galaxy are deficient in heavy elements. We think this is because the little galaxy is not especially crowded, and star formation has occurred quite slowly. As a result there have been, so far, relatively few supernova explosions. Smaller galaxies also have more trouble holding onto the gas expelled by supernova explosions in order to recycle it. Low-mass galaxies exert only a modest gravitational force, and the high-speed gas ejected by supernovae can easily escape from them.
Which elements a star is endowed with thus depends not only on when the star formed in the history of its galaxy, but also on how many stars in its part of the galaxy had already completed their lives by the time the star is ready to form.
Distance to the Galactic Center
The exact distance between the Solar System and the Galactic Center is not certain,  although estimates since 2000 have remained within the range 24&ndash28.4 kilolight-years (7.4&ndash8.7 kiloparsecs).  The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center:
- 7000740000000000000&spades 7.4 ± 0.2(stat) ±&thinsp0.2(syst) or 7020228340141028572&spades 7.4 ± 0.3 kpc ( 7020227057531341939&spades &asymp24 ± 1 kly ) 
- 7020235128631707800&spades 7.62 ± 0.32 kpc ( 7020234626115720003&spades &asymp24.8 ± 1 kly ) 
- 7020237597173772973&spades 7.7 ± 0.7 kpc ( 7020237464334861778&spades &asymp25.1 ± 2.3 kly ) 
- 7.94 or 7020246854206517375&spades 8.0 ± 0.5 kpc ( 7020245978992287100&spades &asymp26 ± 1.6 kly ) 
- 7000798000000000000&spades 7.98 ± 0.15(stat) ±&thinsp0.20(syst) or 7020246854206517375&spades 8.0 ± 0.25 kpc ( 7020245978992287100&spades &asymp26 ± 0.8 kly ) 
- 7020257036942536217&spades 8.33 ± 0.35 kpc ( 7020255439722759681&spades &asymp27 ± 1.1 kly ) 
- 7020268453949587645&spades 8.7 ± 0.5 kpc ( 7020268684745421294&spades &asymp28.4 ± 1.6 kly ) 
An accurate determination of the distance to the Galactic Center as established from variable stars (e.g. RR Lyrae variables) or standard candles (e.g. red-clump stars) is hindered by countless effects, which include: an ambiguous reddening law a bias for smaller values of the distance to the Galactic Center because of a preferential sampling of stars toward the near side of the Galactic bulge owing to interstellar extinction and an uncertainty in characterizing how a mean distance to a group of variable stars found in the direction of the Galactic bulge relates to the distance to the Galactic Center.  
The nature of the Milky Way's bar, which extends across the Galactic Center, is also actively debated, with estimates for its half-length and orientation spanning between 1&ndash5 kpc (short or a long bar) and 10&ndash50°.    Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other.  The bar is delineated by red-clump stars (see also red giant) however, RR Lyrae variables do not trace a prominent Galactic bar.    The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, and most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way. 
The origin of the s-star cluster at the galactic center
(Phys.org) —Scientists Fabio Antonini, of the Canadian Institute for Theoretical Astrophysics, and David Merritt, of the Rochester Institute of Technology, have developed a new theory that explains the orbits of the massive young stars that closely orbit the black hole at the center of the Milky Way.
The discovery of these stars called "S-stars" provided an unprecedented opportunity for studying the black hole at the galactic center itself, but it also raised new questions: how were massive young stars orbiting in a region too violent for them to have formed there? They could not have formed where they are observed because of the strong gravity of the supermassive black hole, implying that they had to have migrated from further out. When theoreticians produced models explaining the migration of the S-stars toward the center the observed orbits didn't match the models. Why were the orbits observed different from what was predicted?
Dr. Antonini is offering the best answers to date for this puzzle in his Thursday afternoon talk at the annual meeting of the Canadian Astronomical Society (CASCA). In "The Origin of the S-star Cluster at the Galactic Center," Antonini is presenting a unified theory for the origin and dynamics of the S-stars.
Explaining how these stars managed to get so close in only tens of millions of years since they formed has been a challenge. "Theories exist for how migration from larger distances has occurred, but have up until now been unable to convincingly explain why the S-stars orbit the galactic center the way they do," Antonini said. "As main-sequence stars, the S-stars cannot be older than about 100 million years, yet their orbital distribution appears to be 'relaxed', contrary to the predictions of models for their origin." Antonini and Merritt's model suggests that the S-stars formed farther out from the galactic center, migrated within their lifetime to the region where they are observed and subsequently attained the observed orbital shapes by interacting gravitationally with other stars near the central black hole.
3-dimensional visualization of the stellar orbits in the Galactic center based on data obtained by the W. M. Keck Telescopes between 1995 and 2012. Stars with the best determined orbits are shown with full ellipses and trails behind each star span
15-20 years. These stars are color-coded to represent their spectral type: Early-type (young) stars are shown in teal green, late-type (old) stars are shown in orange, and those with unknown spectral type are shown in magenta. Stars without ellipses are from a statistical sample and follow the observed radial distributions for the early (white) or late (yellow/orange) type stars. These stars are embedded in a model representation of the inner Milky Way provided by NCSA/AVL to provide context for the visualization. The movie begins at the very center of the Galaxy,
0.015 pc from the supermassive black hole, in the year 1893, and pulls away to a distance of 0.2 pc as the movie reaches the year 2013, ending from the viewing angle of Earth. Credit: (c) University of Illinois
Antonini's and Merritt's research builds on new insights on how stellar orbits at the galactic center evolve due to the joint influence of gravitational interactions with other stars and relativistic effects due to the supermassive black hole.
"Theoretical modeling of S-star orbits is a means to constrain their origin, to probe the dynamical mechanisms of the region near the galactic center and," says Merritt, "indirectly to learn about the density and number of unseen objects in this region."
Supermassive black holes are believed to inhabit the center of most, if not all, massive galaxies. How they form and grow is intimately connected to the formation of the galaxies they inhabit. The black hole in the center of our own galaxy, named Sgr A* (pronounced Sagittarius A-star), is the closest and most extensively studied example. By tracking the orbits of the S-stars over the past several years, astronomers have been able to conclusively show that the object they orbit is indeed a supermassive black hole.