Units to cover: 70, 72,73,74, 75, 76, 78

HomeworkUnit 70, Problems 17, 19

Unit 72, Problem 18Unit 73, Problem 18

Unit 75, Problems 12, 17, 18 19

Star Clusters

• Stars form in large groups out of a single interstellar cloud of gas and dust

• These groups are called star clusters

• Open clusters have a low density of stars – there is lots of space between the cluster’s members

• They can contain up to a few thousand stars in a volume 14 to 40 light years across

• The Pleiades is a very familiar open cluster

Globular Clusters

• Some clusters are much more densely packed than open clusters.

• These globular clusters can have as many as several million stars, in a volume 80 to 320 light years across!

A snapshot of stellar evolution

• Because all stars in a given cluster formed at the same time out of the same cloud of material, we can learn a lot about stellar evolution by examining a cluster’s stars

• We can locate each star in a cluster on an HR diagram and look for the “turnoff point”, the point on the main sequence above which the stars in the cluster have run out of fuel and become red giants

We can deduce the age of a cluster by finding this turnoff point.

The Formation of the Milky Way

• Our galaxy likely began 13 billion years ago as a huge cloud of pure hydrogen and helium, slowly rotating and collapsing

• The first stars formed within this cloud, burning out quickly and violently. This added heavy elements to the cloud

• Population II stars formed next, capturing some of the heavy elements and settling into elliptical orbits around the center of the cloud

• As the collapse continued, a disk formed, and Population I stars formed from the ashes of dying Pop I stars

Galactic Cannibalism

• There are a few observations that are not explained by this model– Some stars follow unusual orbits

in the galaxy– Not all Pop II stars are the same

age– Model predicts that the first stars

might not have been very massive, and should still be around!

• Galactic cannibalism provides some answers– The Milky Way may be absorbing

another galaxy!– Observations show streams of

stars coming from our galaxy’s “victim”.

Composition of Interstellar Clouds

• Light passing through an interstellar cloud can hold clues as to the cloud’s composition

• Atoms in the cloud absorb specific frequencies of starlight passing through, creating absorption lines

• Astronomers can analyze these spectra to determine what the clouds are made of.

• Spectra show that interstellar gas clouds are made of mostly hydrogen and helium, just like the Sun

• Dust particles do not absorb light the same way that gas atoms do, but using similar methods tells us that the dust is made of silicates

Interstellar Reddening

• As starlight passes through a dust cloud, the dust particles scatter blue photons, allowing red photons to pass through easily

• The star appears red (reddening) – it looks older and dimmer (extinction) than it really is.

The Galactic Center and Edge

• Despite the appearance of being closely spaced, stars in the Milky Way are very far apart– At the Sun’s distance from the

center, stellar density is around 1 star per 10 cubic parsecs

• Density is much higher at the core– Exceeds 100,000 stars per cubic

parsec!

• X-ray and gamma ray telescopes reveal a supermassive black hole at the Milky Way’s core– Called Sag A*– 5 million solar masses!

Sag A*

Star Formation in Spiral Arms

A History of Galactic Discovery

• In the early 20th century, the existence of other galaxies was unknown– The Milky way was the Universe!

– Other galaxies were called nebulae

• Light from galaxies always appears fuzzy and diffuse, due to the vast separation between the Sun and the observed galaxy, as well as the separation between the stars of that galaxy!– The paleness of visible light from distant

galaxies is called the surface brightness.

• Galaxies are therefore difficult to observe, even with good telescopes.

More History…

• In the 1700’s, Charles Messier was observing comets, and kept finding objects that while fuzzy, were not comets– He made a list (or catalog) of these undesired

objects, so he could avoid seeing them– They became known as Messier Objects, a

number preceded by an M.– M31 (the Andromeda galaxy) is one such object

• William and Caroline Herschel (1800’s) developed a catalog of faint objects in the heavens– Now known as the New General Catalog– Objects are known by a number preceded by the

letters NGC– Objects can appear in both the Messier and

NGC catalogs!

M31

A Sky Full of Galaxies

• Technology has advanced to the point where we have found as many galaxies as there are stars in the Milky Way!

• Note the gap running along the zero latitude line– Called the zone of

avoidance– Puzzled

astronomers!

The Zone of Avoidance

Dust and the center of our own galaxy merely blocks our view – there is no zone of avoidance!

Distances to other galaxies

• We can use Cepheid variable stars to measure the distance to other galaxies

• A Cepheid’s luminosity is proportional to its period, so if we know how rapidly it brightens and dims, we know much energy it emits

• If we see a Cepheid in another galaxy, we measure its period, determine its luminosity, and calculate its distance!

• Distance between galaxies is huge!– M100 is 17 million parsecs away.

The Sun’s position in the galaxy is

• A. unknown

• B. in the disk of the galaxy

• C. in the spherical halo of the galaxy

• D. in the galactic nucleus

Distances to other galaxies

• We can use Cepheid variable stars to measure the distance to other galaxies

• A Cepheid’s luminosity is proportional to its period, so if we know how rapidly it brightens and dims, we know much energy it emits

• If we see a Cepheid in another galaxy, we measure its period, determine its luminosity, and calculate its distance!

• Distance between galaxies is huge!– M100 is 17 million parsecs away.

Spiral Galaxies

• Spiral arms and a central bulge• Type S

Elliptical Galaxies

• No spiral arms

• Ellipsoidal shape

• Smooth, featureless appearance

• Type E

Irregular Galaxies

• Stars and gas clouds scattered in random patches

• No particular shape

• Type Irr

Galaxy collision and merger

The Mice

• These two interacting galaxies are tidally distorting each other.

Quasars

• Quasars are small, extremely luminous, extremely distant galactic nuclei– Bright radio sources– Name comes from Quasi-Stellar

Radio Source, as they appeared to be stars!

– Can have clouds of gas near them, or jets racing from their cores

– Spectra are heavily redshifted, meaning they are very far away

– Energy output is equivalent to one supernova going off every hour!

• The HST was able to image a quasar, showing it to be the active core of a distant galaxy

Energy Source for Active Galactic Nuclei

• Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths

• A black hole can be both very small, and have an accretion disk that can emit enough radiation

• Likely that at the centers of these galactic nuclei, there are supermassive black holes

• Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus

• There are still many questions to be answered…

The Redshift and Expansion of the Universe

• Early 20th century astronomers noted that the spectra from most galaxies was shifted towards red wavelengths

• Edwin Hubble (and others) discovered that galaxies that were farther away (dimmer) had even more pronounced redshifts!

• This redshift was interpreted as a measure of radial velocity, and it became clear that the more distant a galaxy is, the faster it is receding!

The Hubble Law

• In 1920, Edwin Hubble developed a simple expression relating the distance of a galaxy to its recessional speed.

• V = H d– V is the recessional

velocity

– D is the distance to the galaxy

– H is the Hubble Constant (70 km/sec per Mpc)

• This was our first clue that the universe is expanding!

Which two quantities are shown to be related to one another in Hubble Law?

• A. distance and brightness

• B. distance and recession velocity

• C. brightness and recession velocity

• D. brightness and dust content

Large Scale Structure in the Universe

• Using modern technology, astronomers have mapped the location of galaxies and clusters of galaxies in three dimensions

• Redshift is used to determine distance to these galaxies

• Galaxies tend to form long chains or shells in space, surrounded by voids containing small or dim galaxies

• This is as far as we can see!

How are galaxies spread through the Universe?

• A. They are grouped into clusters that in turn are grouped into clusters of clusters (superclusters)

• B. Galaxies are spread more or less evenly throughout the Universe

• C. They are grouped around our galaxy

• D. none of the above

Seyfert Galaxies

• Seyfert galaxies are spiral galaxies with extremely luminous central bulges

• Light output of the bulge is equal to the light output of the whole Milky Way!

• Radiation from Seyfert galaxies fluctuates rapidly in intensity

Radio Galaxies

• Radio Galaxies emit large amounts of energy in the radio part of the spectrum

• Energy is generated in two regions– Galactic nucleus– Radio lobes on either side

of the galaxy

• Energy generated by energetic electrons– Synchrotron radiation– Electrons are part of the gas

shooting out of the core in narrow jets