The Structure of the Universe

All held together by gravitational forces

Measuring Distance in Space• 1 light year = 9.5 x 1012km or 9.5 trillion• Light speed is 3.0 x 108m/s and light year is

the distance traveled by light in 1 year

Astronomers use light years to measure distance since other units are so large. For example the nearest galaxy is the Andromeda galaxy, which is located at a distance of 21 quintillion km (21,000,000,000,000,000,000 km) away. Such a large figure is difficult to comprehend, when expressed in lightyears, the distance is 2.2 million lightyears, which is much easier to read.

• If you could travel at speed of light you could make 7.5 full rotations around the earth in 1 second

A lightyear in perspective….

How far away are other stars and galaxies when they are light years away?

Pluto at the farthest reaches of our solar system is only 0.0006 light years away.

Light from the sun would take 5.5 hours to reach Pluto, although most of it would be pretty diffused at that point.

Our closest star cluster, Alpha Centauri, is 4.3 light years away. This means that the light emitted from these 3 stars would take 4.3 years to reach us.

We are seeing the past??

• I can take the distance to the Andromeda Galaxy and divide by the speed of light

• Time = distance / speed• That means it takes 2,218,226 years for

the light from Andromeda galaxy stars to reach my eyes….so the stars I see in the sky may already have died out.

• We are seeing what used to be millions or billions of years ago…We see the past.

Nebula

PlanetsStars Other solar bodies

Solar systems

Star Clusters

Galaxies

Galaxy Clusters

Galaxy Superclusters

The Universe

Quasars

Nebula• A massive cloud of gas, dust and heat• The birthplace of stars and planets

Stars• A very large burning ball

of gas• Stars do nuclear fusion

(two small atoms fused to form larger atoms) and produce lots of energy

Planets• “cool bodies”, no

nuclear fusion, no emission of light

• Planet means “wanderer” in Greek b/c wandered in sky among the stars

Small Solar System Bodies

• Comets-frozen balls of gas only visible when near the sun

• Asteroids-small irregular objects of rock and ice

• Meteors = “shooting star” caused by asteroid coming into our atmosphere

• Meteorite – if outerspace rock makes it to Earth’s surface

Solar Systems• An ordered group of

objects orbiting a star (like our sun)

Star Clusters• 10,000 to several million

gravitationally bound stars (2 types)

• Globular clusters are dense spherical shapes

• Open clusters are less dense (only 100 stars or so like Orion Nebula)

• This image to the left is an example of a galaxy cluster

• The galaxies of HCG 87, about four hundred million light-years distant. The large edge-on spiral, the fuzzy elliptical galaxy immediately to its right, and the spiral near the top of the image are members of the group, while the small spiral galaxy exactly in the middle is a more distant background galaxy. Credit: NASA/ESA.

Galaxy• A giant mass of 100’s of billions of stars, nebulae, and interstellar gas and

dust (3 types)• Spiral (and bright) – like ours…THE Milky Way Galaxy or Andromeda

Gal.• Elliptical – most common and are dimmer• Irregular – small and faint

Galaxy Clusterslarge groups of galaxies, often

several hundred, with diameter of 10 million light

years or more• We are part of “The Local

Group” Cluster

Quasars (special galaxy)

• QUAsi-StellAr Radio source • Thought to be blasts of extreme

amounts of energy emitted from supermassive blackhole center of a galaxy

• The brightest and most distant, thus oldest, objects in sky

Galaxy SuperClustersProbably the largest objects in the universe

We are in the Virgo supercluster

The Universe• Defined as everything that physically exists (including dark matter

which is not visible)• Made of over 100 billion galaxies, and “born” from the big bang about 14

billions years ago

Dark matter the hypothetical matter that we can not see, yet is

thought to account for the vast majority of the mass in the observable universe

The observed phenomena which imply the presence of dark matter include the rotational speeds of galaxies, orbital velocities of galaxies in clusters, gravitational

lensing of background objects by galaxy clusters such as the Bullet cluster.