Lesson Objectives:- Halo and disk stars
- How did our galaxy form?
- The black hole at the center of our galaxy
The stars that are in the halo of the galaxy are known as the halo population and have some key differences from the disk population of stars.
As we learned previously, stars in the halo orbit the center of the galaxy with random orientations, while stars in the disk all orbit in the same direction in roughly the same plane.
Halo stars are all old, which means they are all long-lived, low-mass stars. Their spectra show that they contain very little of the heavy elements, as little as 0.02% (point-zero-two percent), indicating they formed early in the galaxy's history.
The cold, dense molecular clouds required for star formation are all in the disk. That is why the disk stars have new stars as well as old stars, and contain a higher proportion of heavy elements.
There are various models of how our galaxy might have formed, but in the simplest model, it began with a protogalactic cloud of hydrogen and helium gas. The earliest stars that formed in this blob of gas were the halo stars. Since they were in random localized regions within the cloud, they would not have had any organized rotation, resulting in the randomly oriented orbits of halo stars. Over time, as this protogalactic cloud continued to collapse, conservation of angular momentum caused the gas to flatten into a spinning disk, and the stars that formed in this disk were the disk stars that shared the organized motion of the disk.
The one problem with this simple model is that if it were true, the oldest halo stars should be the farthest away, and as the cloud compacted, the stars closer to the disk and bulge should be newer and have a higher proportion of heavy elements. Since this has not matched up with observations, another model expands on this simplistic model to propose that several small protogalactic clouds may have merged to form the large protogalactic cloud that became the Milky Way galaxy.
In both models, the interstellar gas settled into the Milky Way's disk, which is why stars no longer form in the halo.
The interstellar medium obscures visible light, keeping us from seeing the center of the galaxy in the visible light spectrum, but radio, infrared, and X-ray telescopes have allowed astronomers to see through those clouds. A variety of evidence points to the existence of a black hole about 4 million times as massive as the Sun at the center of our galaxy.
This evidence includes a unique source of radio emissions coming from the exact center called Sagittarius A-star. Radio emissions also trace out massive magnetic field lines near this center. Orbits of hundreds of stars within a light year of the center point have orbits that indicate a super massive object packed into a relatively small space. A black hole is the only known object that fits the description.