Galaxies in the Universe

Galaxies: From Here to the Horizon Galaxies: From Here to the Horizon

Edwin Hubble (1889 – 1953)American astronomer

Thus the explorations of space end on a note of uncertainty… Eventually, we reach the dim boundary – the utmost limits of our telescopes. There we measure shadows, and we search among ghostly errors of measurement for landmarks that are scarcely more substantial.

WHAT DO YOU THINK?

1. How far away are galaxies?2. Do all galaxies have spiral arms?3. Are galaxies isolated objects?4. Is the universe contracting, unchanging in

size, or expanding?

In this chapter you will discover…In this chapter you will discover… galaxies are categorized by their shapesgalaxies are categorized by their shapes processes that produce galaxies of different processes that produce galaxies of different

shapesshapes galaxies are found in clusters that contain huge galaxies are found in clusters that contain huge

amounts of dark matteramounts of dark matter why clusters of galaxies form in superclusters why clusters of galaxies form in superclusters how some galaxies merge and others devour how some galaxies merge and others devour

their neighborstheir neighbors the universe is changing sizethe universe is changing size

Final Exam Essay QuestionsFinal Exam Essay Questions Essay Questions on the final relating to this Essay Questions on the final relating to this

chapter include:chapter include:What What kinds of galaxieskinds of galaxies are there in the are there in the

universe? How do we measure distances to universe? How do we measure distances to galaxies? galaxies?

What is What is Hubble'sHubble's lawlaw? What does it tell us ? What does it tell us about the evolution of the universe?about the evolution of the universe?

What is theWhat is the missing massmissing mass problem? What problem? What does it mean about the evolution of the does it mean about the evolution of the universe? universe?

Process of ScienceProcess of Science ObserveObserve galaxies at all distances galaxies at all distances ResearchResearch properties, similarities, patterns properties, similarities, patterns HypothesesHypotheses::

Why are they different?Why are they different? When did they form?When did they form? How do they evolve?How do they evolve?

ExperimentsExperiments?? Look over distance (and time!)Look over distance (and time!) Put into a timelinePut into a timeline

SpiralsSpirals

Variations?

SpiralsSpirals

Variations?

Size of nucleus, amount of winding, “fluffiness”

SpiralsSpirals

have a have a disk componentdisk component and bulge & halo and bulge & halo disk contains an ISM of gas & dustdisk contains an ISM of gas & dust relative sizes of bulge/disk & amount of gas/dust vary relative sizes of bulge/disk & amount of gas/dust vary appear white because they contain both blue & red starsappear white because they contain both blue & red stars

Spirals “Edge-on”Spirals “Edge-on”

Galaxies don’t seem to “wind up” over time!

Spiral Arms as Density “Traffic Jams”

Barred SpiralsBarred Spirals

Elliptical Galaxies

only have a only have a spheroidal component;spheroidal component; nono diskdisk very little gas/dust, little active star formationvery little gas/dust, little active star formation appear red because they contain mostly red starsappear red because they contain mostly red stars

Elliptical Galaxy Variations

Size of central nucleusSize of central nucleus Degree of “flatness” or “ellipticityDegree of “flatness” or “ellipticity

Irregular Galaxy Variations

•“none of the above”; neither spiral nor elliptical•appear white & dusty: lots of gas & dust

•have more in common w/ disk component of spirals

•distant galaxies more likely to be irregular•So…more common when the Universe was young

Edwin HubbleEdwin Hubble(1889-1953)(1889-1953)

Discovered Cepheid variables Discovered Cepheid variables in in AndromedaAndromeda galaxy. galaxy.

Calculated distance to Calculated distance to Andromeda Andromeda galaxy.galaxy. 2.2 million light years2.2 million light years notnot in the Milky Way in the Milky Way

Developed a classification Developed a classification scheme for galaxies.scheme for galaxies.

Hubble’s Classification System

Milton HumasonMilton Humason(1891-1972)(1891-1972)

Mule-team driver who Mule-team driver who dropped out of 8th grade.dropped out of 8th grade.

Took spectra & measured Took spectra & measured the redshifts of galaxies.the redshifts of galaxies.

Worked w/ Hubble, who Worked w/ Hubble, who measured the distances to measured the distances to those same galaxies.those same galaxies.

They plotted distance vs. They plotted distance vs. velocityvelocity

The Cosmic Distance Ladder

The Cosmic Distance Ladder

Parallax to Nearby Stars ~500 light years

The Cosmic Distance Ladder

Spectroscopic Parallax to Stars in Galaxy

~30,000 light years

The Cosmic Distance Ladder

Calibrated Variable Stars in Galaxy

~50,000 light years

The Cosmic Distance Ladder

Calibrated Variable Stars, Supernovae in “Nearby” Galaxies

~10 milion light years

The Cosmic Distance Ladder

Calibrated Variable Stars, Supernovae, Most massive galaxies in “Distant” Clusters

~500 million to 5 billion light years

The Cosmic Distance Ladder

Hubble’s Law ~10 billion light years

Cosmic Distance Ladder!Cosmic Distance Ladder!

Hubble’s Law

Distance of Galaxies is indicated by

REDSHIFT of star light.

Hubble’s Law: Velocity of Distant Galaxies Increases with Distance

Closer Galaxies move away from us

Farther Galaxies move away from us faster

Hubble’s Constant indicates AGE of Universe!

Hubble’s Constant Tells Age:

H0 = Velocity/Distance = 1 / Time

Hubble’s Law:

Velocity = H0 x Distance

Galactic “Cannibalism”

More Evidence of Dark Matter

Gravitational Lenses Suggest Dark Matter

Summary of Key IdeasSummary of Key Ideas

Types of Galaxies The Hubble classification system groups galaxies into The Hubble classification system groups galaxies into

four major types: spiral, barred spiral, elliptical, and four major types: spiral, barred spiral, elliptical, and irregular.irregular.

The arms of spiral and barred spiral galaxies are sites of The arms of spiral and barred spiral galaxies are sites of active star formation.active star formation.

According to the theory of self-propagating star According to the theory of self-propagating star formation, spiral arms of flocculent galaxies are caused formation, spiral arms of flocculent galaxies are caused by the births and deaths of stars over extended regions by the births and deaths of stars over extended regions of a galaxy. Differential rotation of a galaxy stretches the of a galaxy. Differential rotation of a galaxy stretches the star-forming regions into elongated arches of stars and star-forming regions into elongated arches of stars and nebulae that we see as spiral arms.nebulae that we see as spiral arms.

Types of Galaxies According to the spiral density wave theory, spiral arms According to the spiral density wave theory, spiral arms

of grand-design galaxies are caused by density waves. of grand-design galaxies are caused by density waves. The gravitational field of a spiral density wave The gravitational field of a spiral density wave compresses the interstellar clouds that pass through it, compresses the interstellar clouds that pass through it, thereby triggering the formation of stars, including OB thereby triggering the formation of stars, including OB associations, which highlight the arms.associations, which highlight the arms.

Elliptical galaxies contain much less interstellar gas and Elliptical galaxies contain much less interstellar gas and dust than do spiral galaxies; little star formation occurs in dust than do spiral galaxies; little star formation occurs in elliptical galaxies.elliptical galaxies.

Clusters and Superclusters Galaxies group into clusters rather than being randomly Galaxies group into clusters rather than being randomly

scattered through the universe.scattered through the universe. A rich cluster contains at least a thousand galaxies; a A rich cluster contains at least a thousand galaxies; a

poor cluster may contain only a few dozen up to a poor cluster may contain only a few dozen up to a thousand galaxies. A regular cluster has a nearly thousand galaxies. A regular cluster has a nearly spherical shape with a central concentration of galaxies; spherical shape with a central concentration of galaxies; in an irregular cluster, the distribution of galaxies is in an irregular cluster, the distribution of galaxies is asymmetrical.asymmetrical.

Our Galaxy is a member of a poor, irregular cluster, Our Galaxy is a member of a poor, irregular cluster, called the Local Group.called the Local Group.

Rich, regular clusters contain mostly elliptical and Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain more spiral lenticular galaxies; irregular clusters contain more spiral and irregular galaxies. Giant elliptical galaxies are often and irregular galaxies. Giant elliptical galaxies are often found near the centers of rich clusters.found near the centers of rich clusters.

Clusters and Superclusters No cluster of galaxies has an observable mass large

enough to account for the observed motions of its galaxies; a large amount of unobserved mass must be present between the galaxies.

Hot intergalactic gases emit X rays in rich clusters. When two galaxies collide, their stars initially pass each

other, but their interstellar gas and dust collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation. The gravitational effects of a galactic collision can cast stars out of their galaxies into intergalactic space.

Galactic mergers occur; a large galaxy in a rich cluster may grow steadily through galactic cannibalism, sometimes producing a giant elliptical galaxy.

Superclusters in Motion A simple linear relationship exists between the distance A simple linear relationship exists between the distance

from Earth to galaxies in other superclusters and the from Earth to galaxies in other superclusters and the redshifts of those galaxies (a measure of the speed at redshifts of those galaxies (a measure of the speed at which they are receding from us). This relationship is which they are receding from us). This relationship is the Hubble law: recessional velocity = Hthe Hubble law: recessional velocity = Hoo x distance, x distance, where Hwhere Hoo is the Hubble constant. is the Hubble constant.

Astronomers use standard candles—Cepheid variables, Astronomers use standard candles—Cepheid variables, the brightest supergiants, globular clusters, H II regions, the brightest supergiants, globular clusters, H II regions, supernovae in a galaxy, and the Tully-Fisher relation—supernovae in a galaxy, and the Tully-Fisher relation—to calculate intergalactic distances. Because of to calculate intergalactic distances. Because of difficulties in measuring the distances to remote difficulties in measuring the distances to remote galaxies, the value of the Hubble constant, Hgalaxies, the value of the Hubble constant, Hoo, is not , is not known with complete certainty.known with complete certainty.

Key TermsKey Termsbarred spiral galaxycluster (of galaxies)elliptical galaxygalactic mergergravitational lensingHubble classificationHubble constantHubble flowHubble lawintergalactic gasirregular cluster (of galaxies)irregular galaxylenticular galaxyLocal Group

poor cluster (of galaxies)regular cluster (of galaxies)rich cluster (of galaxies)spiral density wavespiral galaxystandard candlestarburst galaxysupercluster (of galaxies)trailing-arm spiral galaxyTully-Fisher relation

WHAT DID YOU THINK?Are most of the stars in spiral galaxies

located in their spiral arms? No. The spiral arms contain only 5% more

stars than the regions between the arms.

WHAT DID YOU THINK?Do all galaxies have spiral arms? No. Galaxies may be either spiral, barred

spiral, elliptical, or irregular. Only spirals and barred spirals have arms.

WHAT DID YOU THINK?Are galaxies isolated objects? No. Galaxies are grouped in clusters, and

clusters are grouped in superclusters.

WHAT DID YOU THINK? Is the universe contracting, unchanging in

size, or expanding? The universe is expanding.