Scroll down the page to Explore Stars, Nebulae, Galaxies , Pulsars, Quasars and Black Holes

---- What is a Star? ----

Quite simply, a star is a sun. The closest star to Earth is our own sun. The closest star to our sun is in the Alpha Centauri star system, approx. 4.3 light years away.  Some stars are smaller than our sun, some are much larger.

Globular Clusters are areas of space containing many stars that are densely packed.


Open Clusters are areas of space with many stars that are not as densely packed.  Open cluster stars are younger than Globular cluster stars.

Open Cluster

Globular Cluster

This stellar swarm is M80, one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globualr clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 15 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives.


M80 is also unusual because it was the site of a nova explosion in the year 1860. Nova outbursts occur when a close companion star transfers fresh hydrogen fuel to a burned-out white dwarf. Eventually the hydrogen ignites a thermonuclear explosion on the surface of the white dwarf, giving rise to the nova outburst. The ultraviolet Hubble observations have revealed the hot, faint remnant of this exploding star, which was named T Scorpii in the 19th century.

---- What is a Nebula? ----

A Nebula is an concentration of gases and dust in space. There are different types of nebulae. Diffuse Nebulae are the birthplace of stars. These gases, dust and other matter come together to form stars and star systems. Planetary Nebulae are the remains of stars in the final stages of life, the outer layers of red giants being expelled into space.

This Hubble image captures the infancy of the Stingray Nebula, the youngest known planetary nebula. A ring of gas (green) surrounds the central star with bubbles of gas to the lower left and upper right of the ring. The red lines are bright gas that is heated by a 'shock' caused when the central star's wind hits the walls of the bubble. The nebula is as large as 130 solar systems and is 18,000 light-years away. The Stingray is located in the direction of the southern constellation Ara (the Altar).

In the Large Magellanic Cloud lays the Tarantula Nebula. In the lower right hand corner of this photo is Hodge 301, a cluster of brilliant, massive stars. Many of the stars in Hodge 301 are so old they are exploding as supernovae. Material is blasting out into the Tarantula Nebula at speeds of almost 200 miles per second. Hodge 301 also contains three red supergiants, stars that are close to the end of their lives and about to go supernova.

Looking like stalagmites on a cavern floor, pillars of interstellar dust and gases protrude from the interior wall of a dark molecular cloud. This is an incubator for new stars, the Eagle Nebula, M16. From these gases and materials will form new stars.

This spectacular color panorama of the center of the Orion Nebula is one of the largest pictures ever assembled from individual images taken with the Hubble Telescope. The picture is a mosaic of 15 seperate fields. Located 1500 light-years away along our spiral arm of the Milky Way, the Orion Nebula is located in the middle of the sword region of the constellation Orion the Hunter, which dominates the early winter evening sky at northern latitudes. The stars have formed from collapsing clouds of interstellar gas within the last million years. The most massive clouds have formed the brightest stars near the center and these are so hot that they illuminate the gas left behind after the period of star formation was complete. The more numerous faint stars are still in the process of collapsing under their own gravity, but have become hot enough in their centers to be self luminous bodies.

This is an image of MyCn18, a young planetary nebula located about 8,000 light-years away in the constellation Musca. Features revealed in MyCn18 by Hubble are posing new questions about the formation of planetary nebula.

---- What is a Galaxy? ----

A Galaxy is composed of gas and dust and stars, billions upon billions of stars, all attracted to each other by gravity. There are over a hundred billion galaxies in the universe. The Milky Way galaxy, a spiral galaxy, is home to our Sun and Earth. We are located on the outer part of one the Milky Way's spiral arms. There are about 20 galaxies in a local area of about several million light years around the Milky Way. A very similar galaxy to ours is M31, Andromeda, which can be seen, on clear nights, with the naked eye from Earth.

M31, the Andromeda Galaxy


The Small Magellanic Cloud.  A satellite galaxy to the Milky Way.  Found near the constellation Tucana in the Southern Hemisphere.  At only 200,000 light-years away, the Small Magellanic Cloud is close enough to see in detail with the naked eye.

This group of four galaxies is known as the Hickson Compact Group 87 (HCG 87). The Wide-Field and Planetary Camera 2 on the Hubble Telescope (HST) provides a striking improvement in resolution over previous ground based imaging. In particular, this image reveals complex details on the dust lanes of the group's largest galaxy member (HCG 87a), which is actually disk-shaped, but tilted so that we see it nearly edge-on. Both 87a and its elliptically shaped nearest neighbor (87b) have active galactic nuclei which are believed to harbor black holes that are consuming gas. A third group member, the nearby spiral galaxy (87c), may be undergoing a burst of active star formation. Gas flows within galaxies can be intensified by the gravitational tidal fuel for both active nuclei and starburst phenomena. These three galaxies are so close to each other that gravitational forces disrupt their structure and alter their evolution.


---- What is a Black Hole? ----

A Black Hole is formed when a star over three times the mass of our sun goes supernova and explodes.  The star collapses in on itself with incredible gravitational forces.  What is left is an object that has such a huge gravitational field that it warps space itself.  It sucks everything into it that lays in its area and nothing can escape, not even light.  The only way to see a black hole is by seeing the materials being sucked into it.  Really large black holes, known a supermassive black holes, are found at the center of galaxies.

---- What is a Pulsar? ----

When a star between one and half and three times the mass of our sun goes supernova, the gravitational forces crush the atoms at the core and the star collapses in on itself.  But the forces are not strong enough to form a black hole.  Instead they form a neutron star, where the atoms are composed only of neutrons.  The neutron star starts spinning at a fast rate and shoots off jets of radiation.  We see these jets of radiation here on Earth but because the neutron star is spinning, we only see the jets when they point toward us.  So from our perspective it appears the star is pulsing.  Thus the name pulsar.  But in reality, like when you view a lighthouse, the jets of radiation, similar to the light in the lighthouse, are being shot out all the time.  By measuring the 'pulses' we can determine the speed that the star is spinning.

---- What is a Quasar? ----

When material is being sucked into a black hole, the object being sucked in shoots off radiation.  This radiation is in the form of X-rays, Radio waves and light waves.  We call this jet of radiation a quasar which stand for Quasi-stellar Radio Source.  Before we knew about black holes, astronomers picked up radio waves that seemed like they were coming from nothing (they could not see the black hole).  Eventually this helped lead to discovery of black holes.

Top Left: This radio image of the galaxy M87, taken with the Very Large Array (VLA) radio telescope in February 1989, shows giant bubble-like structures where radio emission is thought to be powered by the jets of subatomic particles coming from the galaxy's central black hole. The false color corresponds to the intensity of the radio energy being emitted by the jet. M87 is located 50 million light years away in the constellation Virgo.

Top Right: A visible light image of the giant elliptical galaxy M87, taken with NASA Hubble Space Telescope's Wide Field Planetary Camera 2 in February 1998, reveals a brilliant jet of high-speed electrons emitted from the nucleus (diagonal line across image). The jet is produced by a 3-billion-solar-mass black hole.

Bottom: A Very Long Baseline Array (VLBA) radio image of the region close to the black hole, where an extragalactic jet is formed into a narrow beam by magnetic fields. The false color corresponds to the intensity of the radio energy being emitted by the jet. The red region is about 1/10 light-year across. The image was taken in March 1999.

Photos Credit: NASA, Hubble, ESO

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