Stellar Evolution Chapter 14

Rare Stars Massive stars are rare for 3 reasons: Large clouds of gas tend to break up into smaller regions and form multiples. Very massive stars are unstable at formation, resulting in eruptions of gas [like a volcano, rather than a bomb] leaving the star and reducing its size. Ex. Eta Carinae (the 2nd brightest star in the sky). Massive stars have short lifetimes.

Note both lobes from 1843 eruption and disk of ejected matter between the lobes from more recent eruption.

Leaving the MS Nest As H is fused into He, the He provides less outward pressure, so the outer layers begin to squeeze the core. The contraction leads to more energy output and the star drifts upward, away from the lower edge of the MS band, called the zero-age main sequence (ZAMS). Note that the sun has moved away from ZAMS. Note: the sun has increased its L since ZAMS, but the Earth has remained at a steady temperature. How? The Earth’s atmosphere has compensated!! Amazing!

The Main Sequence “line” is actually a band, with a starting edge (solid, ZAMS) and an finish edge (dashed).

Numbering our Days… Since stars burn their fuel at a fairly steady rate, we can calculate their lifetimes (T, relative to the sun) with the following (not exact!): For a 10 solar-mass star, this becomes: Or 31.6 M years.

What’s Going on Inside? The interiors of stars greater than 0.4 Msun are not mixed well, so not all of the H is able to be used for fuel. In addition, the He forms “ashes” in the core, which can’t be fused because the temperature is not hot enough. Energy production slows, so the star contracts. This causes a shell around the core to begin fusing H into He.

We’re Movin’ On Up! There are 2 results: a) the fusion shell causes the outer surface of the star to expand into a red giant (thus, moving the star off the MS to the right). b) The He core collapses into degenerate matter, where electrons are forced into the lower energy levels of the gas. This state of matter resists compression (changing temperature has little effect) and is like hardened steel, even though it is still a gas! Electrons are restricted to certain energy levels. The Pauli Exclusion Principle = only 2 electrons per energy level

Core Values The collapse of the core raises its temperature enough so that the He begins fusing into carbon [3 4He  12C] (the triple  process). Low mass stars (0.4 – 3 Msun) undergo a “helium flash,” which is a runaway nuclear reaction in the core. The thermostat is broken because the core is degenerate matter. The core is so small that the outer layers absorb the energy and we wouldn’t see anything.

More Core Values The helium flash is brief, but at its peak produces more energy than 1014 suns! The core’s temperature increases enough so that it stops being degenerate, so the thermostat kicks in again and controls the reactions. Smaller stars never fuse He and more massive stars ignite He before it becomes degenerate—so no He flash.

The Inevitable End Gravity wins in the end—all stars eventually collapse. Table 14-1 on p. 390 summarizes the types of nuclear fuels and products that can occur. Fusion cannot fuse into elements heavier than iron (Fe) because above Fe it loses energy instead of gaining. As new fuels are “burned” in the core, the star adds shells of the prior fuels.

Interior of a Massive Star From Wikipedia; NOT to scale!

Variable Stars A variable star is any star that changes its brightness in a periodic way. One important type was discovered in 1784 by John Goodricke, who was deaf and mute and died at age 21. He noticed that  Cephei changes its brightness by about 1 magnitude over about 5.4 days. This type is called a Cepheid variable star. Also, Polaris. T = 3.97 days.

Variable Graphs

More Variables RR Lyrae variable stars are similar to Cepheids, but with shorter periods (less than a day) and are much fainter. There are 2 types of Cepheids: Type I Cepheids contain similar elements as our sun, while Type IIs are poor in elements heavier than He (as are RR Lyrae variables).  Cephei is a Type I. All of these are used to determine distances of galaxies from Earth.

Spotting Cepheids in M100

How Does This Work? Certain combinations of size and temperature make the star unstable so that it changes size and brightness. The region where this occurs is called the instability strip (Fig. 14-14, p. 385). The pulsation occurs because of a layer of partially ionized He, which acts like a spring to expand or contract the star. Seen as Doppler shifts in their spectra.

The Instability Strip

Variations in Variables’ Periods As stars change their positions on the HR Diagram, they may pass through the instability strip. Some stars temporarily become variable stars. Other stars that are variables actually change their radii and periods. Famous example: RU Camelopardalis (a Cepheid) was observed to stop pulsating for a period of time from 1966-67.

Crossing the Strip Note the sun will not become a Cepheid Variable.