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How galaxies group together

Spiral galaxies are often found in groups of galaxies (up to a few dozen galaxies)

The Milky Way is part of the “Local Group”: more than 45 galaxies grouped mostly around the Milky Way and the Andromeda galaxy (M31). M31 lies approximately 2.6 million light years from the Milky Way.

Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies) Most large clusters have a giant elliptical galaxy at its center

Most large clusters have a giant elliptical galaxy at its center

Most large clusters have a giant elliptical galaxy at its center

Most large clusters have a giant elliptical galaxy at its center

Most large clusters have a giant elliptical galaxy at its center

Most large clusters have a giant elliptical galaxy at its center

What are the three major types of galaxies? How are the lives of galaxies connected with the history of the universe? Galaxies generally formed when the universe was young and have aged along with the universe What are the three major types of galaxies? Spiral galaxies, elliptical galaxies, and irregular galaxies Spirals have both disk and spheroidal components; ellipticals have no disk How are galaxies grouped together? Spiral galaxies tend to collect into groups of up to a few dozen galaxies Elliptical galaxies are more common in large clusters containing hundreds to thousands of galaxies

We measure galaxy distances using a chain of interdependent techniques Distances to galaxies We measure galaxy distances using a chain of interdependent techniques

Galaxies: “island universes” or part of our Milky Way Galaxy Galaxies: “island universes” or part of our Milky Way Galaxy. Do we reside at a special place in the universe or in a fairly average place?

The Puzzle of “Spiral Nebulae”

How do we determine distances to galaxies? bootstrap process involving multiple methods distances to nearer objects used to calibrate distance measures to more distant objects

Standard candle: an object whose luminosity we can determine without measuring its distance. If we can determine the apparent brightness of a standard candle, the we can determine its distance. Luminosity Distance = 4π x Brightness

Cepheid variables: very luminous stars that vary their brightness in a very regular manner. Their luminosity and the period of their variability are linked.

If you can determine the period of a Cepheid variable, then you know its luminosity. We can use Cepheid variables as standard candles Cepheid variable stars with longer periods have greater luminosities

White-dwarf supernovae can also be used as standard candles The precursor stars are all 1.4 solar mass white dwarfs

Apparent brightness of white-dwarf supernova tells us the distance to its galaxy (up to 10 billion light-years)

Hubble and Island Universes

The Puzzle of “Spiral Nebulae” Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, while others maintained they were smaller collections of stars within the Milky Way

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles. Its distance places it well outside the Milky Way, i.e., it is an “Island Universe”

By measuring distances to many galaxies using Cepheids, Hubble found that the velocity of a galaxy towards or away from us and its distance are related in a very special way

The spectral features of virtually all galaxies are redshifted! If the redshift is due to the Doppler effect, then they are all moving away from us v Δλ c λ =

Hubble’s Law: velocity = H0 x distance Redshift of a galaxy tells us its distance through Hubble’s Law: distance = velocity H0 H0 is the Hubble Constant

Distances of farthest galaxies are measured from redshifts

The fact that we see all other galaxies moving away from us does not imply that we are the center of the universe! All galaxies will see all other galaxies moving away from them in an expanding universe. A rising loaf of raisin bread is a good visual model: each raisin will see all other raisins moving away from it as the loaf expands.

The expansion rate appears to be the same everywhere in space The universe has no center and no edge (as far as we can tell)

One example of something that expands but has no center or edge is the surface of a balloon

Cosmological Principle The universe looks about the same no matter where you are within it Matter is evenly distributed on very large scales in the universe No center & no edges Not proved but consistent with all observations to date

Hubble’s constant tells us age of universe because it relates velocities and distances of all galaxies Age = ~ 1 / H0 Distance Velocity

How does the universe’s expansion affect our distance measurements?

Distances between faraway galaxies change while light travels

Distances between faraway galaxies change while light travels Astronomers think in terms of lookback time rather than distance

Expansion stretches photon wavelengths, causing a cosmological redshift directly related to lookback time

How did Hubble prove that galaxies lie far beyond the Milky Way? He measured the distance to the Andromeda galaxy using Cepheid variable stars as standard candles What is Hubble’s Law? The faster a galaxy is moving away from us, the greater its distance: velocity = H0 x distance

How do distance measurements tell us the age of the universe? Measuring a galaxy’s distance and speed allows us to figure out how long the galaxy took to reach its current distance Measuring Hubble’s constant tells us that amount of time: about 14 billion years How does the universe’s expansion affect our distance measurements? Lookback time is easier to define than distance for objects whose distances grow while their light travels to Earth