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AST201H1 Study Guide - Midterm Guide: Helium Flash, Red Giant, Net Force

Astronomy & Astrophysics
Course Code
Michael Reid
Study Guide

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APRIL 18 2012
Cosmological redshift the expansion of the universe stretches out all the photons
within it, shifting them to longer, redder wavelengths
o Tells us how much space has expanded during the time since light from the
galaxy left
An objects lookback time is directly related to its redshift
Lookback time the difference between the current age of the universe and the age
of the universe when the light left the object
Galaxies are moving away from one another; implies that galaxies must have been
closer together in the past
o The periods are very closely related to their luminosities: the longer the
period, the more luminous the star
Obey a period-luminosity relationship
o Once we measure its period, we know its luminosity and can use the inverse
square law for light to determine its distance
Spiral galaxies (Milky Way): disks are filled with cool gas and dust, combined
with hotter ionized gas, and usually display spiral arms
Elliptical galaxies: redder, rounded, often elongated (football shape), contain
very little cool gas/dust but often contain very hot ionized gas
Irregular galaxies: appear neither disk-like nor rounded
Lenticular galaxies: have disk and spheroidal components like spiral galaxies but
lack spiral arms; considered an intermediate class between spirals and
Most large galaxies are spiral; some of the most massive are giant elliptical
Majority of ellipticals are small; they’re the most common type of galaxy
o Have little/no ongoing star formation
Near hot stars we find ionization nebulae ultraviolet photons from the hot
stars can ionize the nebula’s atoms, causing them to glow
Spiral arms must be full of newly forming stars because they bear all the
hallmarks of star formation
Disk population contains both young and old stars
Spheroidal population consists of stars in the bulge and halo; they are always
old and low in mass
Sagittarius A contains a very massive black hole
o By applying Newton’s version of Kepler’s Third Law to the orbits of
these stars they concluded that this object must have a mass of about 4

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million solar masses all packed into a region of space just a little larger
than our solar system
The high-speed gas ejected into space by supernovae or stellar winds
sweeps up surrounding interstellar material, creating a bubble of hot, ionized gas
(atoms are missing some of their electrons) around the exploding star
Milky Way
o Has more than 100 billion stars
o Spiral galaxy because of its spiral arms
o Fairly flat disk surrounding a bright central bulge
o Entire disk is surrounded by a dimmer, rounder halo
o Most bright stars reside in the disk
o Most prominent stars in the halo are found in globular clusters
o 100,000 light years in diameter, disk is only 1000 light years thick
o Sun is located 28,000 light years from the center
o Disk is filled with interstellar gas and dust interstellar medium
Most successful models for galaxy formation:
o Hydrogen and helium gas filled all of space more or less uniformly when the
universe was very young
o The distribution of matter in the universe was not perfectly uniform
The regions of enhanced density originally expanded along with the rest of the
universe; the slightly greater pull of gravity in these regions gradually slowed their
expansion, so they began to contract into protogalactic clouds
o Many galaxies may have formed from the merger of multiple protogalactic
The disk population consists of stars born after the gas in the protogalactic cloud
settled into a rotating disk which is why they all have similar orbits
All galaxies began their lives with gravity pulling matter into a patch of the universe
that was slightly denser than its surroundings, then contracted into a protogalactic
cloud and began to form stars
Explanations for the differences between spiral and elliptical galaxies trace a
galaxy’s protogalactic cloud
o Protogalactic spin significant angular momentum would form a disk =
spiral galaxy; little or no angular momentum would form no disk at all =

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o Protogalactic density high gas density would have radiated energy more
effectively and cooled more quickly allowing more rapid star formation; the
gas could have turned into stars before any of it had time to settle into a disk
= elliptical; lower-density clouds would have formed stars more slowly,
leaving plenty of gas to form the disk of a spiral galaxy
Formation of a gas-rich disk depends on the angular momentum or density of the
protogalactic cloud
A collision of 2 spiral galaxies can create an elliptical galaxy
Tremendous tidal forces tear apart the 2 disks, randomizing the orbits of their stars,
while a large fraction of their gas sinks to the center of the collision and rapidly
Starburst galaxies currently forming stars at a rapid rate; represent a stage of
evolution that many galaxies may have gone through
o The rates of star formation in starbursts are unsustainable
o After its starburst is over, the galaxy presumably returns to its spiral,
elliptical or irregular state
o Starbursts emit strong infrared light because of their interstellar dust
Active galactic nuclei extreme amounts of radiation and sometimes powerful jets
of material emanating from deep in the center of galaxies
o The brightest of these are quasars
Quasars (quasi-stellar radio sources) are another temporary stage in the process if
galaxy evolution
Find them at great distances, so we know that these were most common when
galaxies were in their youth
Quasars in young galaxies must become dormant as their galaxies age
The energy output of a quasar comes from a gigantic accretion disk surrounding a
supermassive black hole
They emit their energy across a wide swath of the electromagnetic spectrum,
radiating approx. equal amounts of energy from infrared to gamma rays; they
produce strong emission lines
Rapid changes in the luminosities of some active galactic nuclei point to an even
smaller size
Occasionally, the luminosity of an active galactic nucleus doubles in a matter of
hours. The fact that we see a clear signal indicates that the source must be less than
a few light-hours across
In a radio galaxy, the active galactic nucleus is the power source and it drives 2 jets
of particles that stream outward in opposite directions at nearly the speed of light
How radio galaxies, quasars, other active galactic nuclei release so much energy
within such small central volumes = the energy comes from matter falling into a
supermassive black hole
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