White Dwarfs

Stellar Death

Here is another representation of the Hertzsprung-Russell Diagram, showing the relation between stellar surface temperature, or stellar class, (x-axis) and luminosity (y-axis) for nearby stars. We are now going to explore the way that a star changes its position on the H-R diagram, over the course of its lifetime. We can divide the stars on the H-R diagram into four groups:

Main Sequence: where most stars are found, running from the lower right hand corner to the upper left hand corner. Stars spend most of their lives on the Main Sequence, in the hydrogen burning phase. Can you find the Sun along the Main Sequence?
Giants: low mass stars which have burned through their hydrogen and moved off (shifting upwards, and to the right) of the Main Sequence.
Supergiants: rare, high mass stars which have burned through their hydrogen and moved off of the Main Sequence.
White Dwarfs: the remnants of low mass (less than ten solar masses) stars which have burned out completely. These are hot, but small, cores of former stars.

Hertzsprung-Russell Diagram. The x-axis is labeled in Spectral Class, and Color Index (B - V), starting on the left with O5 (for which B - V = -0.5), followed by B0, A0 (B - V = 0.0), F0 (+0.3), G0 (+0.6), K0 (+0.8), and finally M0 (+0.9) on the far right) and in Effective Temperature (in kelvin), running from 40,000 on the left to 3,500 on the right. The left side of the plot has a blue background, shading into white, yellow, orange, and finally into red on the right side. The y-axis is labeled in Absolute Magnitude, M_V, starting on the bottom at 15 and ending on the top at -10, and also in Solar Luminosity, starting on the bottom at 0.0001, followed by 0.001, 0.01, 0.1, 1, 10, 100, 1,000, 10,000, and ending on the top at 100,000. In the lower-left corner a fairly broad region labeled White Dwarfs is outlined starting around a spectral class of O5 and an absolute magnitude of 6 and slanting down to A0, 13 and then extending rightward to G0. The positions of well-known white dwarfs Sirius B (B0, 12) and Procyon B (F0, 13) are marked within this region. The somewhat narrower Main Sequence is outline starting around O5, -9 and slanting down across the entire diagram to M0, 15. The positions of well-known Main Sequence stars Achemar (B0, -2), Regulus (A0, 1), Sirius (A0, 2), Altair (A/F, 3), and Sol (the Sun, G0, 5) are marked along the top and middle of the Main Sequence, as well as nearby Barnard's Star and Proxima Cen (M, 14) at the faint end on the right side of the diagram. A broad region labeled Supergiants (I) extends from A0 through M0 at the top of the diagram, with Rigel, Deneb, and Canopus marked around A/F on the blue end and RW Cephei and Betelgeuse marked around K/M on the red end, and delta Cephei and Antares shown similarly just below the supergiant region. A smaller region labeled Giants (II, III) extends similarly around an absolute magnitude of 0, with the positions of RR Lyrae, Aldebaran, and Mira shown from the blue through the red, and a smaller still region labeled Subgiants (IV) appears between the Giants and the Main Sequence. A key point is that as stars evolve off of the Main Sequence into giants they shift up and to the right on the diagram, while white dwarfs are location down and to the left and form a distinct population.

[CSIRO Australia, 2004]

The lifetime of a star (how long it stays burning hydrogen on the Main Sequence) depends almost exclusively on its mass. The more massive a star is, the shorter time it will exist. Even though it has more hydrogen to burn than a lower mass star, because it is more massive and therefore has more pressure in its core, it will burn through its fuel at a much faster rate. As a star nears its end, it begins to burn fuel faster and faster. Because of this, the increased internal pressure pushes the outer atmosphere of the star outwards. This causes giants and supergiants to have very large sizes. They began their lives as normal stars on the Main Sequence, and then as they ran out of fuel they shifted upwards on the H-R diagram (to larger sizes, and slightly lower temperatures).

Stellar lifetimes range from millions to billions of years. A star that has 30 times the mass of our Sun will only live a short million years.
Our Sun will live about 10 billion years. At the end of its lifetime it will expand in size, swallowing the Earth and other inner planets, and becoming a giant star.
Very low mass stars can survive, quietly burning their fuel, for hundreds of billions of years.

How big can stars get? This is a hard question to answer, because the largest, hottest stars are also the most rare. Here is the current record holder, observed with the Hubble Space Telescope in 1999. The young star cluster NGC 3603, located 20,000 light-years across the Milky Way away from us, contains a star called A1. It is 114 times more massive than the Sun, so large that it would encompass the Earth if it replaced the Sun. Burning through its nuclear fuel at a dramatic rate, this star is destined to a short life and an abrupt end.

Is there an upper limit on stellar mass, or should we expect to keep discovering more and more massive objects? Gas can only collapse to form a star if the force of gravity, exerted on all of the particles by each other, is stronger than the local pressure which resists collapse. As stellar mass approaches 150 times that of the Sun this radiation pressure becomes stronger than the gravitational force, and conditions will not allow a stable star to form. Star A1 is thus probably one of the most massive stars we humans will ever find!

Hubble Space Telescope image shows a dense collection of bright, blue and white stars outlined in green, while to the right and below we observe diffuse orange and yellow diffuse clouds of gas.

Galactic nebula NGC 3603, with region containing massive stars outlined in green. [NASA/HST, Brandner, Grebel, Chu]

There are two more forms of stellar remnants which are not shown on the H-R diagram:

Neutron stars, the remnants of high mass stars which have burned out completely.
Black holes, stellar remnants which have collapsed into a small enough space (size) that their gravitational attraction is sufficient to contain all mass, energy, light ... EVERYTHING!