Material from Units 79 -- 86

Our Galaxy, the Milky Way

• A galaxy is a large collection of billions of stars

• The galaxy in which the Sun is located is called the Milky Way

• From our vantage point inside the galaxy, the Milky Way looks like a band of stars across the night sky, with dark dust lanes obscuring the center of the band.

An Early View of the Milky Way

• It is difficult to know exactly what the Milky Way looks like from outside the galaxy!– Similar to trying to figure

out what kind of car you are in, from the inside!

• William Herschel (who discovered the planet Uranus) created a “map” of the Milky Way, based on observations.

• He incorrectly placed the Sun close to the center of the galaxy

The Shape of the Milky Way

Kapteyn’s Universe

• Jacobus Kapteyn improved on Herschel’s view of the galaxy

• Using more modern equipment, Kapteyn attempted to count the number of stars in the galaxy, and estimate their distance from the Sun

• The model was called Kapteyn’s Universe, as the existence of other galaxies was unknown!

• He revised the size of the galaxy to around 18,000 parsecs (18 kiloparsecs, or kpc), again with the Sun near the center

• Both Herschel and Kapteyn were correct in depicting the shape of the galaxy as a disk, with most of the stars lying in more or less the same plane (the galactic plane)

Moving the Center of the Galaxy

• Harlow Shapley used observations of globular clusters to correctly deduce the location of the Sun within the Milky Way

• He reasoned that if the Sun were at the center of the galaxy, then globular clusters would be found in all directions

• He noted that there were more globular clusters found in the direction of Sagittarius than elsewhere

• Therefore, the center of the galaxy must be in the vicinity of Sagittarius!

Today’s view of the Milky Way

• Today we know that the Milky Way is a spiral galaxy approximately 30 kps across.

• The Sun is located around 8 kpc from the center, in one of the spiral arms.

• Most of the stars are concentrated in the galactic plane, or in the central bulge at the center of the galaxy

• Inside the bulge is the nucleus of the galaxy

• Surrounding the disk is a roughly spherical distribution of stars called the halo.

• Globular clusters are distributed throughout this halo, surrounding the center of the galaxy.

The Interstellar Medium

• Space is far from empty!– Clouds of cold gas– Clouds of dust

• In a galaxy, gravity pulls the dust into a disk along and within the galactic plane

• This dust can obscure visible light from stars and appear to be vast tracts of empty space

• Fortunately, it doesn’t hide all wavelengths of light!

The Sombrero Galaxy

Emission Nebulae

• We frequently see nebulae (clouds of interstellar gas and dust) glowing faintly with a red or pink color

• Ultraviolet radiation from nearby hot stars heats the nebula, causing it to emit photons

• This is an emission nebula!

Reflection Nebulae

• When the cloud of gas and dust is simply illuminated by nearby stars, the light reflects, creating a reflection nebula

• Typically glows blue

Dark Nebulae

• Nebulae that are not illuminated or heated by nearby stars are opaque – they block most of the visible light passing through it.

• This is a dark nebula

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

Heating and Cooling in the ISM

• Gas in the ISM is heated by radiation from nearby stars and by stellar winds

• Gas is cooled by re-radiating away energy, especially clouds that are shielded (shadowed) by dust or other cooler stars

• O and B stars are very good at heating, as they put out mostly UV photons

• These UV photons can ionize neutral hydrogen, with two effects:– Causes gas to glow a reddish-pink– Liberated electrons emit radio waves

that can be detected!– These radio waves penetrate dust well,

allowing us to map the galaxy.

The Tuning Fork

• Edwin Hubble organized different galaxy types into a tuning fork shaped diagram

• Ellipticals are labeled E0-E7– E0 is almost perfectly

spherical, E7 is quite flattened

• Spirals are labeled Sa – Sd– Sa galaxies have tightly wound

arms and a large central bulge

– Sd galaxies are loosely wound and have a small central bulge

• Barred Spirals are labeled SBa – SBd– Same flow as the Spirals

Additions to the list…

• Dwarf galaxies (left) are difficult to detect, and may be the building blocks of larger galaxies

• Low Surface Brightness galaxies (above left) are very large, yet very faint galaxies that have very little new star formation occurring

Differences in Star and Gas Content

• Ellipticals:– Low in gas and dust, so contains

mostly older Pop II stars– Contain very high temperature,

very low density clouds of gas that cannot condense into stars.

• Spirals:– Lots of gas and dust, so have

active regions of star formation– Have both Pop II and younger Pop

I stars

• Irregulars: – Many hot, young stars– Large amounts of interstellar

matter– Might be young galaxies

A look back in time

• The Hubble Space Telescope was pointed at a part of the sky that looked empty, taking a 100-hour exposure

• Very distant galaxies were detected, some closer than others

• This technique allows us to see galaxies at various stages of formation

• These early galaxies tend to be smaller than the Milky Way, and to not fall into Hubble’s classification scheme

Galactic Collisions

• Galaxies can collide, though not in the sense of a car accident!• The galaxies pass through one another, and their immense gravitational

pull tears both galaxies apart!• Eventually, a new elliptical galaxy will form…

Galaxy collision and merger

A Ring Galaxy

• If a smaller galaxy plows through the middle of a larger one, a ring galaxy can result!

• Stars are not destroyed, only their orbits are disturbed, redistributing them through the new galaxy

Galactic Mergers

Young galaxies possibly merging to form a larger system

A Picture of the Universe

• This all-sky image gives the positions of over a million galaxies, each with billions of stars…

Our Galactic Neighborhood

• The smallest organization of galaxies are called galaxy groups

• Our local group is called the Local Group

• The Local Group contains 40 known members, including the Andromeda Galaxy and the Large and Small Magellanic clouds, dwarf satellite galaxies of the Milky Way

Members of the Local Group

Rich and Poor Galaxy Clusters

• Rich clusters:– Contain hundreds to

thousands of member galaxies

– Are roughly spherical, with the largest galaxies near the center

– Contain mostly elliptical and type S0 galaxies

– Lots of hot gas and dust

• Poor clusters– Contain only tens of

galaxies– Have a ragged, irregular

appearance– More spiral and irregular

galaxies

Superclusters

• Clusters of clusters are called superclusters– Contain a few to many dozen

clusters of galaxies

– Can be Mpc across!

– The Local Group is part of the Local Supercluster, shown at left.

• The Local Supercluster is heading toward a region of space known as the Great Attractor, where there are a large number of massive superclusters

• There may be super-superclusters!

Missing Mass

• In Unit 73, we calculated the mass of the Milky Way by measuring the orbital velocities of dwarf galaxies in orbit around our galaxy

• We can also count the number of stars in the galaxy, and estimate the galactic mass. The two numbers do not agree!

• Rotation curves do not show the expected decrease in stars’ orbital velocities with distance from the galactic center, so there must be much more mass present in our galaxy

• Astronomers cannot find a large majority of this mass!

• Astronomers call the missing mass dark matter

Many galaxies have flat rotation curves!

Dark matter is not unique to the Milky Way!

Figure 78.03

• 99 percent of the stars in a galaxy are within 20 kpc of the center

• Gas extends far out into the disk, but is not very massive!

• Galaxies are now thought to be embedded in a dark matter halo that surrounds the entire galaxy

• Unfortunately, dark matter cannot be detected directly.

Dark Matter in Clusters of Galaxies

• Missing mass is also a problem in clusters of galaxies!– Not enough visible mass to

hold the clusters together by gravitation, and to keep hot gas in their vicinity

– Cluster mass must be 100 times greater than the visible mass!

– Once again, dark matter seems to be the solution

Gravitational Lenses

• Dark matter warps space just like ordinary matter does

• The path of light rays bends in the presence of mass

• A galaxy or other massive object can bend and distort the light from objects located behind it, producing multiple images

• This is called gravitational lensing

Figure 78.06

Radiation, Matter and Antimatter

• In the first second of the early universe, matter did not really exist; rather, everything was radiation or energy. Cosmologists call this time period the early universe.

• When energy is converted into matter, antimatter is formed as well.

• For a proton-antiproton pair to form, the temperature must be more than 1013 K!

• Matter and antimatter annihilate on contact, releasing energy

• There must have been an asymmetry in the amount of matter and antimatter formed in order for there to be a predominance of ordinary matter today.

Olber’s Paradox

• Over very large distances, galaxies in the universe are more or less uniformly distributed (homogeneous)

• If there are galaxies in every direction, however, why do we not have a fully-lit sky? We should see a star in any direction we look!– This is called Olber’s Paradox

• If there is an edge to the universe, we should be able to see our way “out of the woods”

Olber’s Paradox

A Solution?

• In a sense, there is an edge to the universe, an edge in time

• Light travels at a finite (though fast) speed

• The size of the visible universe is defined as the distance light can travel in the age of the universe

• Galaxies exist at greater distances, but light from them has not reached us yet.

• The edge is called the cosmic horizon

• If we wait long enough, the night sky might become bright!

The Curvature of the Universe

• Remember that mass and energy can curve the space around it.

• As the Universe expands, the distances between the galaxies increases, like galaxies painted on the surface of an inflating balloon

• If the universe was like an expanding balloon (but with the galaxies distributed in three dimensions), travel in any direction would eventually bring you back to your starting place (a closed universe)

Other Possible Curvatures of Space

• In addition to a closed, or positive curvature of space, there are two other options– Space could be flat, or have zero curvature– Space could be curved away from itself, or have negative

curvature– Geometry behaves differently with each curvature!

Measurements of the Curvature of Space

• If space is closed, distant galaxies or clumps of mass will appear larger than they really are

• If space is flat, there will be no apparent distortion in size

• If space is open, distant objects will appear smaller than they really are

• Recent measurements show that space is very nearly flat!

Density of the Universe

• If we can measure the density of the universe, we can predict how much gravitational energy the universe has, and therefore whether it will collapse or keep expanding

• The critical density of the universe, C, is the density at which the total energy of the universe is zero – gravitational energy balances the other two.

M = /C, where is the measured density of the universe

• If M > 1, the universe will recollapse• If M < 1, the universe will expand forever• If M = 1, the universe is exactly at the critical

density

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Supernova Type Ia Findings

• We also need to know how the universe is expanding – this can help us determine the value of M

• We can measure the recession velocity of distant galaxies using Type Ia supernovae as standard candles

• It appears that the expansion rate at a time when the universe was half its current size (z=1) was slower than it is today!

• This shows that the expansion rate is increasing with time! Very puzzling!

Life on Earth

• Life formed on Earth relatively soon after the planet’s formation– For ¾ of the Earth’s history,

only algae and single-celled life forms existed

– Slowly, more complex lifeforms developed

• By 250 million years before the present, dinosaurs and early mammals had evolved.

• Hominids, our distant ancestors, developed 5.5 million years ago

• Homo Sapiens evolved only 500,000 years ago!

Figure 83.04

• Life tends to draw on the substances that are most plentiful: Carbon, Nitrogen, Oxygen and Hydrogen

• Amino acids are organic molecules containing these substances

• Amino acids form proteins, which provide structure and energy to cells

• All life contains DNA – this instruction packet contains all the information needed to build an organism

The Origin of Life

• So how did amino acids form out of the substances available on the early Earth?

• Probably started thanks to complex chemical reactions in the atmosphere and surfaces of Earth

• The Miller-Urey experiment attempted to duplicate the environment of the early Earth

• A variety of complex organic molecules formed in their “atmosphere”

Fossil Eukaryote Algae

The Search for Life on Mars

• It appears that Mars at some point in its history was very much wetter and warmer than it is today

• Scientists have been looking for life there

• The Viking landers (1970’s) tested for the presence of microbes, but returned inconclusive results

• We are still looking!

Fossils of Ancient Martian Life?

SETI

• SETI: Search for Extra-Terrestrial Intelligence

• Listens for electromagnetic evidence of intelligence elsewhere in the universe

• To date, evidence has been sparse.

As a star converts most of its hydrogen in its core into helium, the star gets

a. less luminous and smallerb. hotter and fainterc. more luminous and biggerd. less luminous and red

A hydrogen burning shell is created near the helium core because

a. helium diffuses into the shellb. hydrogen diffuses into the corec. core is hot and densed. both a. and b.

If we observe a star cluster which has all the starsof main sequence present, this cluster is

a. oldb. youngc. was born as a result of supernova explosiond. both a. and c.