Galaxies(And a bit about distances)

This image shows galaxy M 100 in which the Hubble Space Telescope detected Cepheidvariables.

• As their period is related to their luminosity, Cepheid variable stars are reliable standard candles.– If you know the period of

the star’s variation, you can use the period–luminosity relation to learn its absolute magnitude.

– By comparing its absolute and apparent magnitudes, you can find its distance.

Cepheid variable stars

Other ways to measure distance: The Distance Ladder

Supernovae as “Standard Candles”

Edwin Hubble discovered a relationship between a galaxy’s redshift (an indicatorof speed) and its distance.

Like runners that begin at the crack of a gunshot, the fastest move the farthest along the track.

Unlike runners, these galaxies aren’t really moving on their own…the redshift observed is called cosmological redshiftand is due to the expansion of space!

(But this idea came along after Hubble’spioneering discovery.)

• When you look at a more distant galaxy, you look back into the past by an amount called the look-back time.– This is the time in years equal to the distance

to the galaxy in light-years.

Telescopes as Time Machines

Galaxy Types: Spirals

• Spiral arms and contain gas and dust, although some have very little. Their hot, bright and young stars make the arms very luminous, blue in color and easy to see.

• Among spiral galaxies, about two-thirds are barred spirals.

Spiral Galaxies

Galaxy Types: Barred Spirals

Galaxy Types: Ellipticals

These old galaxies have no disk, nospiral arms, and almost no gas and dust.Star formation has ceased. Elliptical galaxies range from huge giantsto small dwarfs.

Elliptical Galaxies

Irregular galaxies may be fragments left over by the merger of larger galaxies.

The table above suggests there are more spirals, but in reality, elliptical galaxies are more abundant and irregulars make up about 25 percent of all galaxies. The luminous young stars of spirals make it much easier to notice them than other galaxy types.

The Selection Effect

Recent studies in the deep universe have revealed blue elliptical and “green pea” galaxies.

Still Much to Learn…

Astronomers now suspect that most elliptical galaxies are formed by the merger of at least two or more galaxies.

Evidence supports that they are often found near the center of galaxy clusters.

The Origin and Evolution of Galaxies

Interacting galaxies can distort each other with tides—producing tidal tails, rings or shells of stars. – They may trigger new star formation

episodes– They may create spiral arms in galaxies

previously without them.– Large galaxies can even absorb smaller

galaxies by merger or acts of cannabalism.

Evidence left inside galaxies reveals that they have suffered past interactions and mergers.

– The Milky Way is a cannibal galaxy—snacking on the Magellanic Clouds (and other

galaxies) as they orbit it. – Its tides are pulling apart

the Sagittarius and the Canis Major Dwarf galaxies.

– This produces great streamers of stars wrapped around the Milky Way.– Almost certainly, our galaxy has dined on other small galaxies in the past.

M82: A Starburst Galaxy

Beautiful ring galaxies and other peculiar features are bull’s-eyes and artifacts left behind by high-speed collisions.

Peculiar Galaxies

Centaurus A – A Galaxy Merger?

The Local Group of galaxies

Bubble and void structure

• In 1943, astronomer Carl Seyfert conducted a study of spiral galaxies. He noted that about two percent of spirals have small, highly luminous nuclei in their bulges.

• Many of these galaxies have powerful radio sources at their centers. This indicates the presence of a supermassive black hole as the power source.These are now called

AGN: Active Galactic Nuclei

Seyfert Galaxies: A type of AGN

• Quasars are the most luminous AGN. • They often look like a star, due to their great distance

(the rest of their galaxy structure can’t be seen).• Many quasars reveal more structure when viewed in

radio wavelengths.

Quasars: Quasi- Stellar Objects

• Quasars represent the earliest observed stages of galaxies. They are the farthest visible objects. The most distant ones we see represent a time when the universe wasvery young, less than abillion years old.

Quasars Through Time

Quasars exhibit strange spectra, showing emission linesin places where no known elements could create them. In 1963, astronomer Maarten Schmidt calculated that if hydrogen Balmer lines were redshifted by z = 0.158, they would fit the observed lines in 3C 273’s spectrum.

Quasars: Mystery Solved

Seeing far away Galaxies: Gravitational Lensing