Galaxies and the foundation of modern cosmology ii
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Galaxies and the Foundation of Modern Cosmology II. What is Hubble’s law?. The Puzzle of “Spiral Nebulae”.

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Galaxies and the Foundation of Modern Cosmology II

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Galaxies and the Foundation of Modern Cosmology II

What is Hubble’s law?

The Puzzle of “Spiral Nebulae”

  • Before Hubble, some scientists argued that “spiral nebulae” were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way.

  • The debate remained unsettled until someone finally measured the distances of spiral nebulae.

Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles.

Hubble also knew that the spectral features of virtually all galaxies are redshifted  they’re all moving away from us.

The percentage of the shift tells us the percentage of the speed of light that the object is moving.

Discovering Hubble's Law

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way.

Hubble’s law: velocity = H0 distance

Redshift of a galaxy tells us its distance through Hubble’s law:

distance =



Distances of the farthest galaxies are measured from redshifts.

We measure galaxy distances using a chain of interdependent techniques.

How do distance measurements tell us the age of the universe?

Thought Question

Your friend leaves your house. She later calls you on her cell phone, saying that she’s been driving at 60 miles an hour directly away from you the whole time and is now 60 miles away. How long has she been gone?

A. 1 minute

B. 30 minutes

C. 60 minutes

D. 120 minutes

Thought Question

You observe a galaxy moving away from you at 0.1 light-years per year, and it is now 1.4 billion light-years away from you. How long has it taken to get there?

A. 1 million years

B. 14 million years

C. 10 billion years

D. 14 billion years

Hubble’s constant tells us the age of the universe because it relates velocities and distances of all galaxies.

Age =

~ 1 / H0



Estimating the Age of the Universe

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 and no edges

  • Not proved but consistent with all observations to date

    This is the fundamental assumption that astronomers make when the model how the universe evolved.

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.


Insert ECP6 figure 15.19

Expansion stretches photon wavelengths, causing a cosmological redshiftdirectly related to lookback time.

Consequences of Hubble’s Law

  • Hubble’s Law (i.e. the expansion of the Universe) is one of the key pieces of evidence we have for the Big Bang.

  • It suggests that the Universe was much denser and hotter in the past.

  • It gives an estimate of the age of the Universe.

Galaxy Evolution

  • How do we observe the life histories of galaxies?

  • How did galaxies form?

  • Why do galaxies differ?

Deep observations show us very distant galaxies as they were much earlier in time (old light from young galaxies).

How did galaxies form?

  • Our best models for galaxy formation assume that:

  • Matter originally filled all of space almost uniformly.

  • Gravity of denser regions pulled in surrounding

  • matter.

Denser regions contracted, forming protogalactic clouds.

H and He gases in these clouds formed the first stars.

Supernova explosions from the first stars kept much of the gas from forming stars.

Leftover gas settled into a spinning disk.

Conservation of angular momentum



So we expect disks to form when galaxies form …

But why do some galaxies end up looking so different?

Why don’t all galaxies have similar disks?

Conditions in Protogalactic Cloud?

Spin: Initial angular momentum of protogalactic cloud could determine the size of the resulting disk.

Conditions in Protogalactic Cloud?

Density: Elliptical galaxies could come from dense protogalactic clouds that were able to cool and form stars before gas settled into a disk.

Distant Red Ellipticals

Observations of some distant red elliptical galaxies support the idea that most of their stars formed very early in the history of the universe.

We must also consider the effects of collisions between galaxies.

Many of the galaxies we see at great distances (and early times) do indeed look violently disturbed.

The collisions we observe nearby trigger bursts of star formation.

Modeling such collisions on a computer shows that two spiral galaxies can merge to make an elliptical.

Collisions may explain why elliptical galaxies tend to be found where galaxies are closer together.

Giant elliptical galaxies at the centers of clusters seem to have consumed a number of smaller galaxies.

Starburst galaxies are probably merging galaxies and form stars so quickly that they will use up all their gas in less than a billion years.

The intensity of supernova explosions in starburst galaxies can drive galactic winds.



The intensity of supernova explosions in starburst galaxies can drive galactic winds.

Galaxy Evolution

  • Mergers play a big role in the evolution of galaxies.

    • There were few elliptical galaxies in the past.

    • Elliptical galaxies are common in dense galaxy clusters.

  • Mergers of spirals probably leads to starburst galaxies that eventually become elliptical galaxies.

    • Starburst galaxies use up all their cool gas in a few hundred million years and then stop forming stars.

    • Dry mergers (no cool gas) between elliptical galaxies will not lead to new star formation (you need spirals with gas and dust for this).

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