Physics 101 - Astronomy - Spring 2019
Class notes for day 28, April 30, 2019
Galaxies
The Milky Way
Our Galaxy has a disk about 100,000 light-years diameter and about 2000 light-years thick (in the vicinity of the Sun), with a high concentration of interstellar dust and gas. It contains around 200 billion stars. Interstellar dust obscures our view into the plane of the galactic disk at visual wavelengths. However, hydrogen clouds can be detected beyond this dust by the 21-cm radio waves emitted by changes in the relative spins of electrons and protons in the clouds. This has allowed astronomers to determine the shape of the spiral arms of the Milky Way, and the interesting result that we live in a Barred Spiral galaxy (previously it was thought to be just a spiral galaxy).
The Milky Way is a disk with at least four bright arms of stars, gas, and dust. These spirals start out from the ends of the bar in the galactic nuclear bulge. Young OB associations, H II regions, and molecular clouds in the galactic disk outline huge spiral arms where stars are forming. The Sun is located about 26,000 light-years from the galactic nucleus, between two major spiral arms. The Sun moves in its orbit at a speed of about 828,000 km/h (or about 500,000 miles per hour) and takes about 230 million years to complete one orbit around the center of the Galaxy.
From studies of the rotation of the Galaxy, astronomers estimate that its
total mass is about 1 x 1012 Solar Masses. Much of this mass is still
undetectable. There is a large amount of matter that we can’t see, the Dark
Matter, which extends out much further than the visible Milky Way. We do not
know what Dark Matter is, except that it is probably not cold gas, dust, or
planets. Various types of particles have been proposed, but none have been
detected in experiments in labs on Earth.
The center, or galactic nucleus, has been studied at gamma-ray, X-ray, infrared,
and radio wavelengths, which pass readily through intervening interstellar dust
and H II regions that illuminate the spiral arms. These observations have
revealed the dynamic nature of the galactic nucleus, but much about it remains
unexplained.
A supermassive black hole of about 4 x 106 Solar Masses exists in the
galactic nucleus. The galactic nucleus of the Milky Way is surrounded by a
flattened sphere of stars, called the nuclear bulge, through which a bar of
stars and gas extends. The entire Galaxy is surrounded by a halo of matter that
includes a spherical distribution of globular clusters and halo field stars, as
well as large amounts of dark matter.
In class, we watched a video about the supermassive black hole at the center
of the Milky Way. You can see it at
http://www.eso.org/public/videos/eso0846a/
The version I downloaded to the classroom PC was larger and better looking than
the version you get from YouTube, but it takes a while to download, depending on
your internet service. You could also download a smaller version from the link
above.
Classification of galaxies
You really need to look at pictures to see how galaxies are classified. The Hubble classification system groups galaxies into four major types: spiral, barred spiral, elliptical, and irregular. Spiral type a, type b, and type c are classified by the tightness of the spiral arms and the size of the bulge. The arms of spiral and barred spiral galaxies are sites of active star formation. According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and nebulae that we see as spiral arms. According to the spiral density wave theory, spiral arms of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB associations, which highlight the arms. Barred spiral galaxies are classified by the tightness of the spiral arms, like ordinary spiral galaxies. Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in elliptical galaxies. As a result, elliptical galaxies look yellowish-white from all the old intermediate-mass stars they contain. There is little blue in the color of ellipticals because all the blue giant stars have died off and no new ones are being formed. Elliptical galaxies have no arms and can be much larger than the Milky Way. They are fairly uniform and can be classified by how elongated they are. Hubble’s “tuning fork” model summarizes the types.
Clusters
Galaxies group into clusters rather than being randomly scattered through the
universe. A rich cluster contains at least a thousand galaxies; a poor cluster
may contain only a few dozen up to a thousand galaxies. A regular cluster has a
nearly spherical shape with a central concentration of galaxies; in an irregular
cluster, the distribution of galaxies is asymmetrical. Our Galaxy is a member of
a poor, irregular cluster, called the Local Group. Rich, regular clusters
contain mostly elliptical and lenticular galaxies; irregular clusters contain
more spiral and irregular galaxies. Giant elliptical galaxies are often found
near the centers of rich clusters.
No cluster of galaxies has an observable mass large enough to account for the
observed motions of its galaxies; a large amount of unobserved mass must be
present between the galaxies of a cluster. Presumably this is dark matter.
When two galaxies collide, their stars initially pass each other, but their
interstellar gas and dust collide violently, either stripping the gas and dust
from the galaxies or triggering prolific star formation. The gravitational
effects of a galactic collision can cast stars out of their galaxies into
intergalactic space.
Galactic mergers can occur; a large galaxy in a rich cluster may grow steadily
through galactic cannibalism, sometimes producing a giant elliptical galaxy.
A simple linear relationship exists between the distance from Earth to galaxies
in other superclusters and the redshifts of those galaxies (a measure of the
speed at which they are receding from us). This relationship is the Hubble law:
recessional velocity = Ho x distance, where Ho is the
Hubble constant. This implies that the Universe is expanding.
Astronomers use "standard candles"—Cepheid variables, the brightest supergiants,
globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher
relation—to calculate intergalactic distances. Because of difficulties in
measuring the distances to remote galaxies, the value of the Hubble constant, Ho,
is not known with complete certainty.
Here is some more about superclusters.
The Virgo supercluster is shown in an interactive map
http://www.atlasoftheuniverse.com/virgo.html
This is only part of a larger structure called Laniakea, discovered in 2014. This larger structure may contain over 100,000 galaxies and is
about 500 million light years across. Here is a short (4 min) video describing
this research:
https://www.youtube.com/watch?v=rENyyRwxpHo A shorter preview video
with different graphics is at
http://vimeo.com/104704518 The observable universe is much larger than
the size of this supercluster. Perhaps 50 thousand superclusters like this could
fit in the part of the universe that we can see.
Homework for today:
http://faculty.wiu.edu/BM-Davies/galaxy_worksheet.html
due before the final exam.
Galleries (I did not have time to show these in class but you might want to look at them if you have a fast connection.)
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Spiral galaxies, edge-on
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http://apod.nasa.gov/apod/ap010510.html
M104 - an Sa galaxy
http://apod.nasa.gov/apod/image/0002/sombrero_vlt_big.jpg
http://apod.nasa.gov/apod/ap000228.html
M65 - an Sa galaxy, tilted
http://apod.nasa.gov/apod/ap960812.html
NGC 891 - an Sb galaxy
http://apod.nasa.gov/apod/ap020703.html
NGC 5907 - an Sc galaxy
http://apod.nasa.gov/apod/ap080619.html
NGC 253 - an almost-sideways Sc galaxy
http://apod.nasa.gov/apod/ap030525.html
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Spiral galaxies, face-on
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Sa galaxies
NGC 1357
http://www.noao.edu/outreach/aop/observers/n1357block.jpg
Sb galaxies
M81
http://www.noao.edu/image_gallery/images/d2/02283.jpg
M63
http://apod.nasa.gov/apod/ap000627.html
Sc galaxies
M51
http://apod.nasa.gov/apod/ap060219.html
M74
http://apod.nasa.gov/apod/ap071201.html
http://apod.nasa.gov/apod/ap110406.html
NGC 300
http://apod.nasa.gov/apod/ap020821.html
NGC 2997
http://apod.nasa.gov/apod/ap020804.html
Barred Spiral Galaxies
M95
http://apod.nasa.gov/apod/ap070314.html
M83 (is like the Milky Way) SBb
http://apod.nasa.gov/apod/ap070724.html
http://apod.nasa.gov/apod/ap991206.html
NGC 6217
http://apod.nasa.gov/apod/ap091228.html
NGC 2442
http://apod.nasa.gov/apod/ap100325.html
http://apod.nasa.gov/apod/ap090228.html
NGC 4731
http://apod.nasa.gov/apod/ap100429.html
NGC 1672
http://apod.nasa.gov/apod/ap070418.html
NGC 1300
http://apod.nasa.gov/apod/ap060827.html
http://apod.nasa.gov/apod/ap080622.html
http://apod.nasa.gov/apod/ap050112.html
NGC 1365 SBc
http://apod.nasa.gov/apod/ap030413.html
NOAO Gallery:
http://www.noao.edu/image_gallery/
2dF galaxy redshift survey - large image
http://magnum.anu.edu.au/~TDFgg/Public/Pics/2dFzcone_big.gif