introduction to galaxies - stony brook astronomy€¦ · introduction to galaxies ... •note that...
TRANSCRIPT
Introduction to Galaxies
• Basic Unit of the Universe = Galaxy
• Galaxy - gravitationally-bound aggregate of
a) 106-1011 stars
b) Gas (few 10s of % by mass)& dust (Mgas/Mdust ~ 200-500)
c) Dark matter
• Note that stars form from gas, so the gas content ofgalaxies must have been higher in the past
Galaxies
• Composition = stars & gas are ~ 70% H, ~ 25% He, ~few % metals by mass
• Masses ~ 106 - 1012 Msun (1 Msun = 1.99x1030 kg)
• Sizes ~ 10 - 100 kpc
• Rotation Period of Galaxies ~ 10 - 100 Myr (I.e., theones that do rotate!)
• Average separation ~ 1 Mpc
Age of the Universe – Hubble Diagram (1926)
• v (km s-1) the Doppler motion Δλ / λ = v / c• R (Mpc), where f ~ R-2.• So, v ~ R, v = H0R
Age of the Universe –Hubble Diagram
(1926)
• H0 = 50-100 km/s/Mpc (we’re adopt H0 = 75 km/s/Mpc)• tH = R/v = 1/H0 = 1-2x1010 yr ago (~ Age of Universe)• Age of Earth = 4.5x109 yr• Note: We’ve ignored acceleration/deceleration for this
calculation
Typical spiral galaxy - Milky Way
• Number of stars ~ 1011
• Mass ~ 1012 Msun
• How many times has the Sun orbited the galactic center? Distance of Sun from the center of the galaxy ~ 8.5 kpc Time for one orbit t = 2πR / v = 2π 8.5 kpc / 250 km s-1 = 2x108 yr Thus, the Sun has made 4.5x109 / 2.0x108 ~ 20 turns
around the galactic center
How often do stars collide?
• The Number density of stars in the disk is,
• The mean cross section, σ, of stars is calculated byassuming every star is like the sun,
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Volumedisk = (thickness)π (radius)2 = HπR2
= (3×1019m)π (3×1020m)2 = 8.5 ×1060m3
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n =Nstars
V=
1011stars8.5 ×1060m3 =1.17 ×10−50m−3
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σ = π (2Rsun )2 = 6.08 ×1018m2
Mean freepath λ =1nσ
=1.4 ×1028km
Stellar collisions (continued)• Given that vrandom = 40 km s-1,• Stars collide every,
• I.e., not very often• Note that considering the gravitational cross section only
lowers this time by a factor of 100.• The benefit of this is that we can treat stars as pointmasses, & galaxies as a continuous mass distributionresponding to the gravitational field in the galaxy.
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tcollision ~λ
vrandom=1×1019 years
Stars• In actuality, every star is not like the sun. Stars range in
mass from 0.1 Msun to 100 Msun.• Objects with M < 0.1 Msun cannot ignite• Objects with M > 100 Msun would be blown apart by
radiation pressure
Interstellar Gas & Dust
• Normal ISM has gas number density n = 105 - 106 m-3 &temperature T ~ 100 K
• Dust has a size a ~ 0.1 µm (10-7 m) & a refractorycomposition (metals - e.g. SiO2)
Disk Gas Depletion Rate of the Milky Way• SFR ≈ 3 Msolar yr -1• M(H2) ≈ 2.5x109 Msolar• Depletion time = 2.5x109 / 3 = 8x108 yr
• The Sun takes 2x108 yr to orbit the galaxy, thus depletion timewill occur in 4 revolutions
• The universe is 15 Gyr, thus depletion time takes 5% the age ofthe universe
• Most spiral galaxies have several x108-9 Msolar of molecular gas
• Why does the MW have 2.5x109 Msolar of molecular gas? Whynot 0?
• Given the rapid depletion time & the age of the universe, whereare the dead disks (I.e., spiral galaxies with no gas in theirdisks)?