AstronomyTopic 06:

Galaxies

Stephen’s Quintet

Galaxies

Galaxy: A group of millions or billions of stars, gas, and dust held together by gravity

Our sun and all visible stars are members of our galaxy, the Milky Way

Our Galaxy

The Milky Way is visible as a band of light crossing the sky during summer and winter

Represents the concentrated light of the billions of stars in our galaxy

At night, we witness the view from within our galaxy

Milky Way Galaxy

A disk of stars that contains 100-200 billion stars

100,000 light years in diameter, about 3000 LY thick

The center of the Milky Way (nuclear bulge), swells to about 10,000 LY thick

Milky Way Diagram

http://zebu.uoregon.edu/~imamura/123/images/mw-schematic.jpg

Milky Way

Infrared Milky Way (COBE)

http://www.astro.psu.edu/users/niel/astro1/slideshows/class21/slides-21.html

The view from within, in IR light

Sun and Galaxy

Sun located about 2/3 distance away from the center (30,000 LY)

The entire Milky Way rotates around its center

Sun orbits the center of the galaxy at 563,000 mph

One galactic revolution takes 220 million years

Sun’s Revolution

http://www.envirotruth.org/images/graphics/suns_path.jpg

Milky Way, a Spiral Galaxy

Spiral galaxy, most are concentrated in a central nucleus

Spiral arms of stars wind outward from the nucleus

M101

Milky Way Map

http://members.nova.org/~sol/chview/milkyway.jpg

Star Clusters: Open Clusters

A few dozen to thousands of stars loosely bound together by gravity

Found mainly in the galaxy’s disk and spiral arms

More than 1000 have been discovered in the MW

Young stars that recently formed from nebulosity

Jewel Box open cluster (NGC 4755)

Star Clusters: Globular Clusters

Huge, concentrated balls of thousands to millions of stars

Found in galactic halo, a spherical region centered on the nucleus

Contain the oldest known stars

About 150 globular clusters have been discovered

Omega Centauri, globular cluster (NGC 5139)

Cluster Summary

Type Open Cluster Globular Cluster

Stars Dozens to thousands (Population I)

Hundreds to millions (Population II)

Location Within galactic disk

Orbit galaxy in halo

Age Young, thousands to millions of years

Among oldest known stars (billions of years)

Open Cluster HR Diagrams

•This HR diagram plots the stars for many open clusters

•As clusters age, their main sequence stars appear lower on the HR diagram

•Pleiades are younger than M67

Open Clusters

Pleiades—M45M67

Which cluster is oldest? Note the star colors of the main stars in each cluster

Globular Cluster M55

Colors indicate temperatures, red (cool) to blue (hot)

The “turn off” area on the main sequence represents the cluster’s age

Dark Matter

Galaxies consist of ordinary stars, gas, and dust

Observed motions of stars in a galaxy indicate unseen (dark) matter

Due to dark matter, galaxies rotate faster than the observable matter allows

Visible galaxies surrounded by large, nonluminous galactic halos

http://www.pparc.ac.uk/Ps/aac/images/image33.gif

Dark Matter in NGC 3198

http://bustard.phys.nd.edu/Phys171/lectures/ngc3198_opt_rad_rot.jpg

Between the Stars

Space between the stars in a galaxy is a vacuum, or empty space

Also termed interstellar mediumMatter between the stars is of gas and dustInterstellar matter 99% gas, 1% dustGas consists of 75% hydrogen and 25% heliumInterstellar dust similar in size to cigarette smokeIn spiral galaxies, gas and dust is concentrated

in the disk and spiral arms

Molecular Clouds

> 100 interstellar molecules have been detected in molecular clouds between stars

Water vapor, amino acids, and other organic molecules (urea, alcohol)

Ingredients of life can be found in the galaxy’s molecular clouds

Cosmic origin of life?

Nebulas

Historically, a nebula meant any type of hazy, glowing patch of light in the night sky

Andromeda Galaxy once termed the Andromeda Nebula

Today, nebula means interstellar gas and dust

Emission Nebulas

Emission nebulas glow due to excitement from nearby stars

Also called diffuse nebulas or HII regionsOrion Nebula (M42), famous example of

an emission nebula (next slide)

Pillars of Creation (M16)•In this iconic photo from the Hubble Space Telescope, a small portion near the center of M16, the Eagle Nebula, is revealed

•Note the pink, newly formed stars within the nebula

Dark Nebulas

Dark nebulas, or absorption nebulas, molecular clouds that block the light of distant stars

E.E. Barnard used photography to map the galaxy’s dark nebulas in the early 20th century

Dark nebulas today bear “B” numbers in his honor

Barnard’s S Nebula

http://www.astropix.com/HTML/D_SUM_S/B72.HTM

A dark nebula represents interstellar dust that blocks the light of stars from behind

Horsehead Nebula

Famous dark nebula (B33) located near the Belt of Orion

Star Populations

Walter Baade (1944) divided a galaxy’s stars into two categories:

Population I Hot, young, and luminousFound in galactic diskContain heavy elements beyond H and He

Population IIOld, cool stars, some first stars in universeFound in globular clusters and galactic nucleusAlmost all H and He

Catalogs

Messier Catalog of 110 objects (1784) by French astronomer Charles Messier

Dreyer’s New General and Index Catalogs of thousands of objects (1895-1908)

NGC contains 7840 objects“M” or “NGC”, or “IC” numbersObjects, mainly galaxies, star clusters,

and nebulae

Messier Catalog

All of the Messier objects are shown in this montage

M1 starts the upper left, M110 ends the lower right

Formation of the Milky Way

Milky Way appears to be about 13 billion years old

MW likely represents one of the original galaxies created after the Big Bang

Sun and planets are 4.5 billion years oldBig Bang origin of universe 13.7 bya

Other Galaxies

Until 1924, Milky Way was the “universe”Edwin Hubble proved that galaxies other than

our Milky Way existHubble used Cepheid variable stars to measure

the distances to galaxiesHubble also photographed stars in the

Andromeda “Nebula”The universe contains an estimated 100 billion

galaxies. Each of these galaxies contains about 100 billion stars

Standard Candles

An astronomical object whose absolute magnitude is know from its observed characteristics

Can be used to determine distances to galaxies up to 10 million LY from earth

Examples: Cepheid variable starsGlobular clustersSupernovas

Andromeda Galaxy

Our nearest large neighborSimilar in size and shape to Milky WayAndromeda Galaxy is visible to the naked

eye as a glowing spot in the constellation of Andromeda

2.3 million light years distant (wave!)

Andromeda Galaxy (M31, M32, M110)

Andromeda Constellation

Locate M31 and M33

Triangulum Galaxy (M33)

M33

The pink glow of emission nebulas can be seen in this photo

Virgo

Virgo Supercluster About 200 galaxies

are visible in small telescopes

Galaxies range from Ursa Major through Coma Berenices to Virgo

Sombrero Galaxy

Center of Virgo Cluster

Virgo Cluster near M84, M86

M86

Sombrero Galaxy (M104) from HST

Sheets and Voids

Superclusters are located in thin sheets of galaxies

Sheets border empty voidsVoids resemble gigantic bubblesThe universe resembles a sponge

Large-Scale Structure of the Universe

http://www.astro.princeton.edu/~cen/PROJECTS/p1/DARKallz0.jpeg

Sheets & Voids:http://heasarc.nasa.gov/docs/cosmic/sheets_voids.html

Active Galaxies (AGN)

An active galaxy emits large amounts of energy from its nucleus

AGNs exhibit jets of hydrogen racing outward along an axis

Massive black holes?Mysterious quasars might represent

extremely active galaxies

Galactic Cannibalism

Galaxies can merge together to form larger galaxies

Elliptical galaxies may be the result of multiple galaxy collisions

NGC 2207 (left) and IC 2163 (HST)

Quasars

Resemble faint stars, are strong radio sourcesAcronym for Quasi-Stellar Radio SourceSmall objects, yet emit energy egual to

thousands of galaxies combinedMost distant and powerful objects knownMight represent early stage of galaxy growthSeyfert galaxies are active galaxies with nuclei

that resemble quasars

3C 273-Quasar

HST image of 3C 273 showing disk details (quasar blocked)

3C 273 Starfield in Virgo

3C 273 is one of the first identified quasars, it resembles an 11th magnitude star in Virgo and is visible in small amateur

telescopes