Scholars Online Astronomy - Chapter 19: Main Sequence Stars
Reading: Astronomy, Chapter 19: On and after the Main Sequence
- Section 1: The rate of and degree of hydrogen fusion depends on the mass of the star. Stars under 0.4 solar masses undergo sufficient convection that there is no separate core; hydrogen fusion continues until the star runs out of hydrogen. Stars over 0.4 solar masses burn out hydrogen at the core; helium fusion begins in the core while hydrogen fusion begins in a shell above the core and no fusion begins at further distances from the center of the star.
- Section 2: In higher mass stars, as the core converts to helium fusion, the star becomes a red giant. Remember that luminosity depends on the total flow of energy (flux) through the surface of the star. A star can have low per-area flux but high total flux when it expands. The intense energy output also destroy planets in orbit around the (new) red giant.
- Section 3: In stars between 0.4Mθ and 3Mθ, Pressure is not initially high enough to trigger helium thermonuclear reactions. As the core compresses, it warms up, the atoms become completely ionized, and the electrons crowd together introduce a kind of counter pressure. This resistance is called electron degeneracy, and prevents expansion of the core as it heats up, until the pressure is great enough to overcome even electron pressure, there is a helium flash, and the core becomes stable.
- Section 4: With stars exhaust their hydrogen and begin burning helium, they undergo enough changes that they no longer conform the characteristics of a main sequence star. We find them instead on the red giant branch. Studies of globular and open clusters allow us to create models of stars with identical initial compositions but different masses. Cluster ages are given by the turnoff point, where members of the cluster move off the main sequence onto a giant branch.
- Section 5: Observing stars has allowed us to identify two populations of stars. Population I stars are metal-rich, born of the clouds of expelled gases of an earlier generation of stars, usually found in the arms of the galaxy and open clusters. Populations II stars are older, metal-poor stars, often found in globular clusters.
- Section 6: Alls stars pulsate to some degree, bound between the opposing forces of thermal expansion and gravitational collapse. Certain older stars have a wider range of pulsation between full contraction and full expansion, so they actually change signification in brightness. Cepheid variables have long-term periods from 1 to 100 days that depend on the innate luminosity of the star. The period-luminosity relationship allows astronomers to calculate the intrinsic brightness of Cepheids, and determine their distance by direct observation of their apparent brightness. RR Lyrae variables are population II stars that can be used in the same way to determine the distance to globular clusters.
- Section 7: In close binary systems with overlapping gravitational fields, matter can begin to flow from one star to the other. This changes the stellar lifecycle of the stars, with the originally smaller star becoming more massive than its partner over time.
Key Formulae to Know
|Helium Triple-α reaction
||He + He → Be
He + Be → C
He + C → O
|Main Sequence Lifetime as a function of Mass
f: fraction of stars mass converted into energy by hydrogen fusion
M: total mass of star
c: speed of light
Read the following weblecture before chat: Stars in Mid-Life
Planetarium Program: Use your planetarium program to explore stars at different stages.
- Center on the Ptolemy cluster and adjust the sky display to about 12° x 10°. Use the status options to turn on the H-R diagram for all the stars in the picture. Adjust the H-R diagram options to display the diagram and star plots using spectral type.
- Chose 5 stars and hover over each of them. Two stars should be in the cluster. The other three stars should be definitely not in the cluster (chose large stars of different colors.) From the information popup of each star note the
- Spectral Class
- Location on the HR diagram.
- Estimate the star's age from the data you've collected.
Website of the Week: Read about the Main sequence stars at the Australia Telescope National Facility site.
- Required: Complete the Mastery exercise with a passing score of 85% or better.
- Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!
Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.
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