Astr 1020 1st Edition Lecture 8Current LectureJessica (Duane E122): Mon/Wed 2:30-3:30Shannon: Tues 12:15-2:00Q: What is the common trait of all main sequence stars?A: They all generate energy through hydrogen fusionHow big is it?- What is its radius?- Luminosity depends on both Temperature and Surface Area- Hotter stars are brighter per unit area (T^4), but surface area also matters (R^2)The star Aldebaran (eye of the bull) is 500x more luminous that the Sun. Yet it appears as a bright orange star in our sky. How can this be?L-Aldebaran =500LsunT –Aldebaran =3900KT –Ald/ Tsun =3900K/5800K =.7R^2 L/T^4 (Rald/Rsun)^2 = (Lald/Lsun)/(Tald/Tsun)^4 = 500/.7^4Rald= 45 Rsun- Main Sequence stars: 0.1-10Rsun- Giants are 10-100 Rsun- Supergiants are really huge- White dwarfs are .01 Rsun (Earth sized!)What is the possible mass range for stars?- Mass governs core temperature, need T>5 million K to fuse HHe- Smallest mass to generate this temperature is about 8% of Sun’s mass (.08 Msun)o Brown Dwarf –M<.08 Msun “star” doesn’t generate energy via fusion. Very dim and cool. Hard to detect - High mass end? Unclear, but probably ~120 MsunAnything larger will blow itself apart as soon as it formsMain Sequence Star SummaryHigh Mass: High Luminosity, short lived, large radius, hot, blue, very rareLow Mass: Low Luminosity, long lived, small radius, cool, red, very commonWhy are some stars not on the Main Sequence?Not fusing HHe- Stars near the ends of their livesGiants, Supergiants and Dwarfs- Temperature is same or lower than Sun- Luminosity is much higher- Must be enormous - White Dwarfs: much hotter than Sun, but much less luminous (dense) - Must be tinyHow old is it? Stellar lifetimes are too long to watch stars actually ageLooking at collections of stars: Star Clusters- Cluster stars are all ar the same distance- Cluster stars are all about the same age- Range of different mass starsOPEN CLUSTERS- 100s to 1000s of stars- loosely bound together by gravity (if at all)- usually young – older clusters are pulled apart by galactic tides- Why aren’t there old open clusters? GLOBULAR CLUSTERS - 100,000 to millions stars- tightly bound into a ball, Stars are very dense in core- Ted to be quite old (but not always)Q: You observe spectra of all the stars in a star cluster and discover that there are many G stars like the sun, but no O or B stars. How old is the star cluster?A: 1-10 billion yearso Stars do NOT move along the main sequence as they liveo (Highest mass stars leave main sequence first) o ***Main-sequence turnoff” of a cluster tells us its age*** o EVERYTHING DEPENDS ON MASSSTELLAR TOOLBOX• Distance - parallax…good to nearby stars, cluster MS fitting or other methods for more distant stars • Luminosity - infer from measured apparent brightness and distance • Temperature - spectral classification (OBAFGKM) • Radius - infer from luminosity and temperature (L=R2T4) • Mass - watch something orbiting the star - only works for binaries. Masses of single stars mustbe inferred from spectral type• Age - cluster ages can be determined from MS turnoff. Can’t say anything for single
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