“I love the atmosphere / I love that I can breathe / I love the planets / Oh, Pluto, please don’t leave / We love the universe / We’ll stop the dwarf jokes now / Boom de yada, boom de yada/ Boom de yada, boom de yada”
As we have now begun working towards deeper and deeper fields, we have realised that we can go no further without discussing something of incredible importance: stellar evolution. There are two main types of stellar evolution: high mass and low mass. We’ll be discussing low-mass evolution this time—the stars that don’t go bang.
First, let’s define what a “low mass” star is. The technical definition means any sun 4 M⊙ (M⊙ stands for “mass of the Sun”, and the dot-in-circle thing is the symbol for sun) or less. While these stars are actually not all that small, they are very low mass in comparison with the high-mass stars that do go bang.
These stars start out as any other—as part of a collapsing cloud of gas that becomes a protostar (Figure 1). Any protostars with masses less than about 0.08 M⊙ never get hot enough to begin nuclear fusion. As we mentioned last week, these are known as brown dwarfs (Figure 2), and anything smaller than a brown dwarf is considered a planet.
For all other stars in the protostar stage, their core temperature will eventually hit the critical temperature to begin nuclear fusion—10 million K. At this point, the star quickly becomes stable and enters into the main sequence (Figure 3). For a low-mass star, the Main Sequence stage takes billions or even trillions of years, and is where the star spends most of its lifetime. Hydrogen fusion through the P-P chain or the CNO cycle is constant and efficient, and for the most part, the star is very stable.
Now this is where the differences in high mass and low mass stars begin. Low mass stars of less than 0.5 M⊙ will never begin helium fusion after their hydrogen fusion supply runs out. These are red dwarfs (Figure 4), and they simply cool off and fade away after billions and billions of years, leaving behind just a white dwarf. The reason they can stay on the main sequence so long is because hydrogen fusion proceeds at a slower rate since the star has less mass. Also, especially for lower-mass red dwarfs, heat transport by convection keeps helium from building up at the core, allowing the star to burn a larger proportion of its hydrogen. Red dwarfs are the most common kind of star in the universe, but are very hard to detect because they emit so little radiation. Our nearest star neighbour, Proxima Centauri, is a red dwarf.
Slightly higher-mass stars of 0.5-10 M⊙ will actually become red giants instead of dying a slow, boring death like the extremely-low mass stars. What happens in these stars is that after fusion in the core stops fusing hydrogen to helium, the star contracts, allowing the temperature and pressure to rise high enough for a hydrogen-burning shell to ignite. This is the subgiant branch. The helium core keeps growing until it reaches something called the Schönberg-Chandrasekhar Limit, which causes the core to contract rapidly. After this, the gravitational energy released from the core causes the immediate outer layer of the star to expand a bit and the temperature drops a bit as well. The helium core continues to contract, and it heats the hydrogen shell above it, where fusion speeds up. This increased rate of fusion means more energy is released, which makes the star become anywhere from 1,000-10,000 times brighter. The layers also expand a lot due to the radiation pressure. This, friends, is the stage that we call red giant (Figure 5). During the red giant branch, a convection zone forms near the surface and eventually reaches down nearly to the core, transporting material from the interior to the surface—this is called the first dredge-up (the second and third dredge-ups occur later, during the helium-burning phases).
This is where things start to become really interesting—that is, if you’re a larger star. Once hydrogen fusion goes on long enough, and temperature (about 108 K) and pressure become high enough, there will be a brilliant helium flash to start helium to carbon fusion—if the mass is low enough (under 1.4 M⊙). If not, then helium fusion will begin fairly quietly, for only in lower-mass stars does the helium core become degenerate before fusion ignites. Fusion occurs in the core through the triple alpha process, although the hydrogen shell is still burning around it. This allows the star to move onto the Horizontal Branch, where its equilibrium is mostly restored and it burns relatively quietly. However, helium runs out faster than hydrogen did. Now the star enters the Asymptotic Giant Branch, with an inert carbon-oxygen core and two shells burning helium and hydrogen. The AGB is dubbed “Asymptotic” because the star now has a higher temperature and its path on the HR diagram seems to use the original path on the Red Giant Branch as an asymptote.
However, low-mass stars don’t have enough mass to create the pressures and temperatures required to fuse carbon, so even as the core contracts, no further fusion ignites there. Helium fusion is very, VERY dependent upon temperature—a small difference can lead to a large difference in energy output. Along with strong stellar winds, these thermal pulses cast off the outer layers of the star, which will become a planetary nebula (Figure 6).
Now, all that is left of the original star is the nebula and a white dwarf (Figure 7), which is just a compressed core of hot carbon supported by electron degeneracy pressure, with a little bit of other elements like oxygen or neon. Eventually the white dwarf will cool off and become a black dwarf when it no longer radiates a significant amount of heat. However, it takes so long for white dwarfs to cool off that astronomers have not yet observed a single black dwarf, and they are really only theoretical objects right now.
Thus ends the sad life of a low-mass star.
TL;DR: The life of a low mass star starts in a gas cloud which then becomes a protostar. If it’s a really small star, it turns into a brown dwarf. If it is larger, it starts nuclear fusion and becomes a main sequence star. Billions of years after this, the main sequence star will turn into a red giant. Larger stars will start helium fusion after the red giant phase, and eventually expel their outer layers. These outer layers turn into planetary nebulae, and the core becomes a white dwarf that eventually cools to form a black dwarf.
http://cosmos.phy.tufts.edu/~zirbel/ast21/handouts/StellarEvolution.PDF (the last page of this has some really nice HR diagrams with evolution tracks)
Carroll and Ostlie, An Introduction to Modern Astrophysics, 2nd ed. (p. 447-451, 457-474)
So last week we talked about how new stars are formed from collapsing clouds of gas and dust, and how these protostars make their way towards the Main Sequence. This week, we’ll go over some of the different kinds of Pre-Main Sequnce objects as well as other things related to early stellar evolution that we find interesting.
T Tauri stars and their higher-mass counterparts Herbig Ae/Be stars are temperamental things, perhaps somewhat like toddlers (or teenagers). These young stars are still accreting matter from a thick surrounding disk and spewing it out in high velocity jets. The rotating disk is formed through conservation of angular momentum, since the initial collapsing cloud had at least a little bit of rotation to it, and therefore the speed of rotation increases as the cloud collapses. The jets produce Herbig-Haro objects, which are luminous patches of stuff moving away from a protostar. Material ejected from the star collides with ISM and causes it to glow brightly.
We know that T Tauri stars are young stars because of the amount of lithium they still contain in their atmospheres, since lithium is burned up quickly and cannot exist in such large quantities in old stars. They’re also irregular variables, with random changes in luminosity over a matter of days (mood swings, perhaps?). In fact, they’re so irregular that unlike basically every other kind of varstar, it’s impossible to classify stars as T Tauris based on their light curves and astronomers have to resort to looking at their spectra instead. From spectra, we can also distinguish two classes of these stars. Classic T Tauri stars (cTTs) have large accretion disks and show strong emission lines, while weak T Tauri stars (wTTs) hardly have any disk.
T Tauri spectra often show something called a P Cygni profile, a blueshifted absorption line right before an emission line (typically of hydrogen-alpha), first discovered in Luminous Blue Variable (LBV) P Cygni. This is a sign of mass loss, since the absorption line shows that light from the star is being absorbed by gas in front of it, and its blueshift means that the gas producing it is moving towards us, and therefore away from the star itself. The broad emission peak results from the fact that the star is expelling matter in all directions, and so some of it appears blueshifted to us and some of it appears redshifted.
FU Orionis stars, or FUors (yes, they really are called that), are T Tauri-like stars that undergo sudden increases in mass accretion. Matter from the inner disk falls onto the star, both causing the disk to shine so brightly that it outshines the star itself and creating extremely high speed winds. It is thought that all T Tauri stars go through several FU Orionis “temper tantrums” before settling down on the main sequence.
To put it bluntly, brown dwarfs are slightly pathetic “failed stars” with masses less than 0.072 Msun but greater than that of a gas giant planet such as Jupiter. They simply don’t have enough mass to ignite the hydrogen fusion reactions necessary to form a main sequence star. Brown dwarfs generate energy mostly through the Kelvin-Helmholtz mechanism of gravitational contraction or through the burning of elements such as lithium or deuterium. We shouldn’t make fun of them too much, though, since there are a huge number of brown dwarfs in our galaxy and they’re extremely hard to detect (although Spitzer has discovered quite a few of them through observations in the infrared), so you never know where one may be lurking…
On the other end of the star-mass spectrum, there are things called OB associations and superbubbles (sadly you cannot have the fun of popping them). OB associations are associations of O and B-type (i.e. high mass) stars that have similar radial or kinematic motions, forming what’s called a kinematic group. They are an association also because they generally have similar ages, forming from the same collapsing gas cloud. This goes into the study of stellar kinematics, which is relatively broad and won’t be fully mentioned, but it should be appropriately fascinating for all you evolving stars, I mean, readers out there. Superbubbles come from high stellar winds associated with OB associations. The reason they form is that multiple winds and shock waves from supernovae can form bubbles in a sort of spherical shape as they spread out from the star, and then combine with other bubbles to form superbubbles.
Some more fascinating features to our lovely early stars are circumstellar disks and protoplanetary disks (propylds). Very simply, the circumstellar disk forms around a protostar as it spins and accretes a disk of material. A propyld is when that disk is, as the name indicates, possibly able to form a planet. So yes, we came from a bunch of spinning, hot stuff in space. To quote Carl Sagan and others, “we are star stuff.” We are literally star stuff, yes, be happy for our stellar-ness and the fact that everyone’s relative is the Sun. Moving on from our little side rant of how awesome space is, these disks are major hints as to protostars, their development, and how binary stars or planets are formed.
Well, we’ve gotten this far. Yes, we’ve discussed quite a lot about the variety of ways that a protostar forms, and what it’s influenced by. So, we have reached the Zero-Age Main Sequence (ZAMS)! This may sound odd, but it makes quite a bit of sense. It refers to the line across the H-R diagram for masses when a star is formed, or when it is at age zero on the main sequence (see what we did there). There is in fact an inverse relation between star formation and time it takes to form. This can reveal the Initial Mass Function (IMF), which basically defines again star formation. It shows that most stars form with lower masses due to fragmentation and other reasons that make massive star formation more difficult.
TL;DR — T Tauri stars are young stars still in the process of accreting matter, which they may spew out in jets to form Herbig-Haro objects. They often have P Cygni profiles in their spectra, which show that they are ejecting matter. FUors are thought to be T Tauri stars that suddenly have matter dumped onto them from the disk and increase greatly in brightness. Brown dwarfs are “failed stars” without enough mass for the fusion of hydrogen to helium to take place. A group of massive stars may be part of an OB association, which may in turn be home to a superbubble. Circumstellar disks and protoplanetary disks are… well, exactly what they sound like. Once a protostar has stabilized, it reaches the Zero-Age Main Sequence on the H-R diagram. The Initial Mass Function illustrates the formation of stars of different masses.
Sources and links for further reading:
- http://tinyurl.com/fuorionis (html version of the PostScript file http://www.ifa.hawaii.edu/~reipurth/reviews/hartmann.ps)
In addition to this list, see last week’s sources/links.