Physics 101 - Astronomy - Spring 2019

Class notes for day 25, April 18, 2019

Lives of stars

Interstellar medium

The interstellar medium is the material between the stars, and is gas and dust. Dust grains are about 10^-7 m in diameter, about the size of smoke particles. Dust causes reddening of the light that passes through it, but it does NOT redshift the light, so the absorption lines stay in the same place. This reddening is due to absorption (and scattering) of the blue components of light (and even more-so in UV). In some ways this is similar to the reddening in Earth’s atmosphere; as the light of the Sun passes through a long distance of air, it is more reddish and when it hits clouds, they appear with the reddish colors of the sunset. You don't see this during the middle of the day because sunlight comes almost straight down through the atmosphere (a shorter distance than during sunset) and so has all the colors of the visible spectrum and is white (so clouds are usually white).

The interstellar gas is very dilute, about ONE atom per cubic centimeter (on average). In some places it is much denser. The distribution of gas is very uneven, however. It is mostly Hydrogen (90%), with some Helium (9%), and a small fraction is everything else (1%).

We looked at a number of pictures that show gas or dust clouds.

In a detail of the Trifid nebula, there is a pillar of cold molecular gas, and you can see a jet coming out of a hidden star, which is about 0.5 parsec long. See the figure in the book or Powerpoint.

In Emission Nebulae, ultraviolet light causes hydrogen to glow with a pinkish color. The pictures are often in false colors, though, so remember that these emission nebulae almost always look pinkish to the human eye.

There are many images of emission nebulae at http://hubblesite.org/images/news/33-emission-nebulae

The Eagle Nebula has the "pillars of creation" in false color, from the Hubble Space Telescope. This is one of the most famous pictures ever taken in Astronomy. It show a star-forming region and by looking at the detail, various protoplanetary disks can be seen. There are also EGGs - evaporating gaseous globules.

The second half of the lecture is about star formation and evolution through a star's long stay on the main sequence.
Star formation occurs in seven steps:

1 – an interstellar cloud
2 – shrinking cloud fragments
3 – a fragment is the size of our solar system
4 – protostar center reaches 1,000,000 K
5 – protostar at about 10 solar radius, 4000 K surface temperature
6 – ignition of fusion in core (now at 10 million K), now a star
7 – reaches main sequence after a period of stabilization

Atomic motions in a big cloud of gas are rarely influenced by gravity. They just keep colliding and coming apart. When there are enough atoms, molecules, and particles of dust in a cloud to equal the mass of the sun, however, and the temperature is about 100 K, the entire cloud can start to shrink due to its own weight, and we get stage 1 of star formation. (The “collapse” of a cloud is probably “triggered” by some event in nearby space, like a supernova or a planetary nebula that tends to push the cloud and cause part of it to shrink.)

Stage 2: Cloud Fragmentation probably occurs. Fragments may contain one to several solar masses of molecular gas and dust.

Stage 3: The cloud continues to shrink until it is about the size of our solar system. By now it is starting to have a disk-like shape and is rotating.

A protostar can be plotted on the H–R diagram after reaching stage 4. It is heated solely due to contraction and is fairly cool, but might be 1000 times as luminous as our Sun, mostly because of its huge size; it has a lot of area of surface that radiates light. It might be a big red-hot sphere about the size of Mercury's orbit.

Newborn Star on the H–R Diagram:

Stage 5 – T Tauri stage – has violent surface activity and may form “jets” (see the image in the text)
Stage 6 – the core reaches 10 million K and finally we get fusion of hydrogen and the star is powered by nuclear fusion energy, not just heat from contraction
Stage 7 – the star reaches the main sequence and maintains a stable size and luminosity for a long period of time.

Prestellar Evolutionary Tracks for stars of other masses are similar to the example of our own Sun, but shifted; see the figures in the book or the powerpoint.

The minimum mass needed to get nuclear fusion and produce a real star is about 0.08 solar mass, or about 80 times the mass of Jupiter. With less mass all we get are “brown dwarfs”, which are “failed stars” that never ignite the hydrogen fusion reactions that power ordinary stars. The one shown in the book has a mass of about 50 times that of Jupiter.

We believe that most stars form in clusters from a single large cloud that fragments into pieces. An example of an Open Cluster is the Pleiades cluster (M45, a.k.a. “the seven sisters”). These are called Open Clusters because the stars will eventually drift apart.

H-R diagram of Globular Cluster: These are different, they may have a million stars. An example in the book is the Omega Centauri globular cluster, which contains no main sequence stars with mass greater than 0.8 solar mass, so it is over 10 billion years old. The stars in a globular cluster stay in the cluster for long periods of time (billions of years) because of their mutual gravitational attraction, so they are not called open clusters (which can disperse in several tens of millions of years).

Life on the main sequence - low to medium mass stars

Stars will remain on the main sequence for most of their lifetimes. Solar composition changes during the time on the main sequence. During stage 7, hydrogen burning causes a build-up of helium in the star’s core. At a certain point, the star will begin to change and no longer stay on the main sequence.

Stars with Masses between 0.08 and 0.5 times the mass of the Sun have low core temperatures, live a long time, convect helium from the core, so it mixes uniformly, and will end up composed entirely of helium. Some of these could live a hundred times longer than the present age of the Universe, so they are just beginning their lifetime on the main sequence.

A G-Type Star is similar to our Sun. For this discussion, we follow the evolution of a star like the Sun, with one solar mass. The core of the star builds up the product of fusion of hydrogen, which is helium, until the core is mostly helium. Then, hydrogen shell burning occurs around an “ash” core, which is mostly helium, and the temperature is T = 10 million K. The hydrogen shell burning causes higher pressure on the envelope, which causes the star to expand into a Red Giant. The star follows the yellow curve on the H–R diagram (see the book or the PowerPoint).

Stage 8 is the “subgiant branch” and the radius is about 3 times the solar radius. An example is the star Arcturus, with M = 1.5 M_solar and R = 23 R_solar, and it has a luminosity about 100 times solar. Arcturus is now expanding into a red giant.

Stage 9 is very short, maybe just a few hours, called the helium flash, when helium burning begins suddenly, but not so explosively that the star is severely affected. The Helium Flash is like a huge nuclear explosion of helium “flashing” or burning quickly into carbon at 10⁸ K.

Stage 10 follows this helium flash and lasts much longer. During helium burning, three He nuclei combine to produce one carbon nucleus plus other products. The position of the star on the HR diagram is the Horizontal Branch, where the star stays for a medium-long period while it burns helium. Re-ascending the Giant Branch occurs in a way similar to the original move up to a giant. Burning in the H and He shells is even faster than before, so the star expands even more on this “asymptotic branch”.

We will have the 4th exam next Thursday, April 25.

Late news item: newly acquired images of dusty disks around young stars have been released; see this article:
http://www.eso.org/public/news/eso1811/