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Stars, clusters, supernovas, nebulas, galaxies:

What are Messier and Other Objects?


NASA Hubble image of star WR124 in 1998
STAR WR124 in 1998 Hubble image
click to enlarge [nasa]

A star is a sphere of hot glowing gas, a single ball of fire in space, often millions of miles in diameter. Stars differ in size, brightness and color. The material in some stars is 10,000 times as thin as Earth's air at sea level. On the other hand, some stars are so dense a cupful of material would weigh tons if it could be brought down to Earth. Inside, stars have temperatures measured in millions of degrees. At their surfaces, temperatures up to 55,000 degrees are common. Stars are a long way off, yet all stars visible from Earth, even with telescopes, are within our own Milky Way galaxy. Proxima Centauri, the star nearest Earth, is so far away it's only a pinpoint of light in the largest telescopes on Earth. Astronomers now think many stars may be ringed by planets and other small bodies, just as our Sun has its Solar System of planets, moons, comets and asteroids. Planets reflect starlight, while stars shine with their own light. Earth is a moist rock circling a star in the outer arm of a spiral galaxy we call the Milky Way. We refer to our star as the Sun. We live on one of nine planets which are accompanied by moons, comets, asteroids, gas and dust in a collection of stuff circling our star. We refer to this stuff and the Sun as our Solar System. The Sun is one star among 100 billion stars in the Milky Way, which is a flat disk-shaped spiral galaxy 100,000 lightyears in diameter with a bulge at the center, like a huge blazing pinwheel floating through space.

details > Stars [Hubble]
details > Archive of Latest Photos [Hubble]
details > Best of Hubble [SEDS]
details > The Brightest Stars [SEDS]
details > Naming Stars [SEDS]

NASA Hubble image of Phi Persei binary star system in 1997
PHI PERSEI binary star system in 1997 Hubble image
click to enlarge [nasa]

Astronomers suggest that half or more of all of the stars we see in the sky actually are star systems that we refer to as double stars or binary stars. There also are multiple star combinations composed of three or more stars. Double and multiple stars orbit around a common center of gravity. Not all combinations of stars that orbit each other in deep space combine in the same way. Sometimes, a bright star has a faint star as its companion. In other instances, both stars are equal in magnitude. Astronomers also see optical double stars, which are two stars that look like they are close together, but actually are far apart. These stars appear close together in the sky only because they lie along the same line of sight. One member of an optical pair is much farther away from Earth. The brightest star in a multiple star system is the primary or A star. Fainter stars in the group are called companion stars and are labeled with the letters B, C, D, etc. It sometimes is possible for observers on Earth to see each star in a binary separately by using seven-power binoculars.

details > Binary Stars [Hubble]
details > Binary and Multiple Stars [SEDS]
details > Double Stars [SEDS]
details > Stars with Planets [Hubble]
details > Winnecke Catalog of Double Stars [SEDS]

NASA Hubble image of Globular Cluster M15 in 1995
GLOBULAR CLUSTER M15 in 1995 Hubble image
click to enlarge [nasa]

When many stars congregate, we call the large-scale object they form a cluster. Astronomers recognize two different kinds of clusters – open and globular. Open clusters, also known as galactic clusters, can contain from only a handful of stars to more than a hundred new stars being born in a single cloud of hydrogen gas and cosmic dust. The clusters are held together by gravity. Globular clusters, on the other hand, are tightly packed balls of thousands or millions of old stars. Astronomers find them forming a halo around the central hub of our Milky Way galaxy. The gobular clusters probably were formed from an earlier generation of stars and so are very old. Estimates of their ages range from 12 to 16 billion years. Our Milky Way galaxy has about 200 globular clusters flying in long oval orbits that take them far outside the galaxy. Other galaxies also have have been seen to have globular cluster systems, sometimes containing thousands of globular clusters.

details > Star Clusters [Hubble]
details > Star Clusters [SEDS]

NASA Hubble image of Star V838 Monocerotis in 2002
STAR V838 MONOCEROTIS in 2002 Hubble image
click to enlarge [nasa]

Variables are stars that can be seen to change in brightness over time. Before there were telescopes, stargazers thought their nighttime views were of a permanent nature. While thy saw the planets, the Sun, and the Moon move, and comets come and go across the sky, the stars seemed to remain constant. The fixed stars were seen as a source of stability above an unstable world. There were a few exceptions, of course. There was the occasional rogue star which seemed to misbehave, stimulating awe or fear. For instance, a star seen in the constellation Cetus, the Whale, sometimes was bright and easily seen. At others, it became very dim. Ancient stargazers named it Mira, which means wonderful. Another star was named Algol and thought of as a demon star. It was the mythological evil eye of Medusa's decapitated head in the constellation Perseus. Ancient stargazers imagined Algol winked at them as its light varied over a three day period. Today, telescopes tell us that Mira and Algol are variable stars. Now we have a catalog of thousands of variable stars. Their fluctuations in brightness range from only a fraction of one magnitude up to ten or more full magnitudes. Astronomers have identified several different kinds of variable stars &ndash pulsating, erupting, and eclipsing. The brightness that a star appears to exhibit is its apparent magnitude. From our perspective on Earth, a star's magnitude depends on its distance from Earth and how bright it really is – its absolute magnitude. Astronomers plot changes in brightness over time on a graph called a light curve.

details > Variable Stars [Hubble]
details > Variable Stars [Chandra]
details > Variable Stars [SEDS]
details > The First Known Variable Stars [SEDS]

NASA Hubble image of Cygnus Loop supernova in 1995
CYGNUS LOOP supernova in 1995 Hubble image
click to enlarge [nasa]

A supernova is the explosive death of a massive star. In 1054, a guest star was seen by the Chinese. It was a supernova explosion witnessed in the area of Earth's sky known as the constellation Taurus where, much later, astronomers would find an expanding gas cloud they would call the Crab Nebula. In 1572, Danish astronomer Tycho Brahe observed a supernova, changing everything by demonstrating that celestial bodies are not unchanging. Astronomers have wondered for a long time why certain supernova explosions create magnificent nebulas, yet leave no spinning pulsar at the center. This abnormal situation has been a problem for astronomers trying to calculate star births and deaths and the ages of galaxies and the Universe.

details > Supernovas [Hubble]

NASA Hubble image of Egg Nebula in 1997
EGG NEBULA in 1997 Hubble image
click to enlarge [nasa]

Nebulas are shadowy clouds of gas and dust lurking in the spiral arms of our Milky Way galaxy. There are emission nebulas, reflection nebulas, dark nebulas, and planetary nebulas: Supernova remnants are the dust and gas remains of very violent stellar explosions.

details > Nebulas [Hubble]

NASA Hubble image of Cartwheel Galaxy in 1995
CARTWHEEL GALAXY in 1995 Hubble image
click to enlarge [nasa]

A galaxy is a vast island of stars floating through the Universe – a cloud of millions or billions of stars attracted to each other and held close by their gravity, while floating as a group through the Universe. There seem to be more galaxies strewn across the Universe than grains of sand on a beach. Astronomers have seen galaxies of many different shapes and sizes. Most common are flat pinwheel spirals, egg-shaped ellipticals, and oddly-shaped irregulars. The galaxies most widely known are Andromeda Galaxy, Large Magellanic Cloud galaxy and, of course, our own Milky Way galaxy While a Milky Way may be a candy bar, the Milky Way is a collection of hundreds of billions of stars traveling through the Universe together like a giant cloud floating across the sky. Our own star is the Sun. It is just one among the 750 billion to one trillion stars that astronomers say compose the Milky Way galaxy. If you think about it, that's a whole lot of stars. To top that, there are untold trillions or quadrillions or quintillions of galaxies beyond our own. Each of those galaxies also is a giant cloud of stars traveling through the Universe. Imagine how many stars there must be across the whole Universe! Our Solar System is a set of nine planets plus asteroids and comets surrounding our star, the Sun. Most of the hundreds of billions of other stars in the Milky Way may have their own solar systems. For an individual star of the Milky Way to be seen distinctly in Earth's night sky, it must lie sufficiently close to our Solar System to be discerned separately. The Milky Way is a flattened spiral galaxy that spins. You might imagine it flying through space like a giant Frisbee at a speed of more than a million miles an hour. Astronomers have seen several different galaxy shapes: Astronomers believe they see an extraordinarily powerful object – a black hole – at the center of the Milky Way.

details > Galaxies [Hubble]
details > Galaxies [SEDS]

NASA Hubble image of Seyfert's Sextet galaxy group in 2002
SEYFERT'S SEXTET galaxy group in 2002 Hubble image
click to enlarge [nasa]

Galaxies cluster together just as stars cluster together. Clusters range from a few dozen galaxies to several thousand. The galaxies are held in their cluster by gravity. The big cluster of galaxies nearest to us is the Virgo Cluster. Our own galaxy, the Milky Way, is a member of a smaller group of galaxies known as the Local Group. It contains three large galaxies and more than 30 smaller galaxies. In turn, the Local Group and other nearby groups of galaxies are part of the so-called Local Supercluster, or Virgo Supercluster, which is dominated by the big Virgo Cluster. The powerful gravity of the big clusters like Virgo attract small groups and individual galaxies and small clusters in their neighborhood. As that happens, the big clusters grow and heat up, which causes them to expand. Astronomers don't know yet whether our Local Group will eventually be swallowed by the Virgo cluster.

details > Galaxy Clusters [Hubble]

NASA Hubble image of Quasar 3C273 in 2003
QUASAR 3C273 in 2003 Hubble image
click to enlarge [nasa]

In the 1960s, astronomers came across some extremely bright, very small objects that apparently are as far away as an object can be across the Universe. They named the newly-found objects quasi-stellar radio sources, or quasars for short. Today, most astronomers think quasars are the highly luminous hearts of distant galaxies. No one knows how they produce their vast energy but they apparently are up to a thousand times brighter then the average galaxy – possibly 10 to 1,000 times brighter. They seem to be extraordinarily small, maybe only one or two lightyears in diameter. That would compare with our own Milky Way galaxy, which is 100,000 lightyears in diameter. Quasars are receding from the Earth. In fact, calculations suggest that quasars are moving away from us at speeds approaching that of light and that their distance from the Earth is enormous – as far away as 14 billion lightyears at the horizon of the known Universe. Quasars radiate prodigious amounts of energy in the form of X rays, ultraviolet rays, radio waves, and other forms of electromagnetic radiation. Astronomers wonder if the energy source might be gas spiraling into a massive black hole at the heart of the distant galaxy.

details > Quasars [Hubble]

Objects in Messier Catalog
Open Star Clusters26
Globular Clusters29
Spiral Galaxies28
Elliptical Galaxies11
Irregular Galaxy1
Diffuse Nebulas7
Planetary Nebulas4
Supernova Remnant1

From 1758 to 1782, the French astronomer Charles Messier cataloged 110 celestial objects that he worried might be mistaken for comets. His catalog includes galaxies, star clusters and nebulas in 36 different constellations seen in the night sky. Among the best known are the Andromeda Galaxy (M31), the Orion Nebula (M42), and the Pleiades (M45) star cluster. France's King Louis XV nicknamed Messier the Comet Ferret. Messier's friend Pierre Méchain contributed some 30 star findings to the catalog.

details > Messier Catalog Object Images [SEDS]
details > Messier Catalog Object Images [IPAC]
details > Messier Catalog Object Images [DSS]
details > Messier Catalog of Nebulae and Clusters [OBSPM]

Constellation Families
Bayer Group
Heavenly Waters
Hercules Family
La Caille Family
Orion Family
Perseus Family
Ursa Major Family
Zodiacal Family

When we look into the night sky,the stars seem to form groups. We call the large groups constellations. The mythical constellation named Ursa Major – the Big Bear – contains many stars including a few that seem to form a shape of an object that we call the Big Dipper. Another well-known constellation is Orion. For most people, the first step to understanding a map of all of the sky is to learn the names of the 88 constellations that compose all of the sky. The stars in the constellations are not necessarily close to each other in space. For example, the middle five stars of the Big Dipper are relatively close to each other, but the first and last stars only look like they are in the same group. Actually, they are much farther from Earth than the middle five. In fact, they are moving slowly away from each other. Some stars the we see in the constellation Orion are relatively close together, but one bright red star we see in the constellation, Betelgeuse, is much nearer to Earth than the others in the constellation. The 88 constellations have been divided into eight constellation families. Mankind first gave names to stars and groups of stars in the days of the Babylonians and Chaldeans, 2000 to 3000 years ago. Constellations were named by ancient sailors as they observed the stars for navigation. Desert people also used stars to find their way. Each culture saw different groups in the sky and named them after familiar objects on Earth. Most of the constellation names we use today came from the ancient Greeks who thought they saw the figures of their gods and heroes in the sky. Greece is in the Northern Hemisphere and they only named those they could see. Other regions of the night sky around the celestial south pole were named by astronomer Johan Bayer who honored the tradition of the ancient Greeks by using names connected to the sea. French astronomer La Caille named the last 13 to cover sparsely-populated regions between the existing constellation. He named most of the 13 after science instruments.

details > Table of Constellations [SEDS]
details > Constellation Families [SEDS]

BeehiveBelt of OrionBier
Big DipperBull of PoniatowskiCirclet
CoalsackFrederick's GloryGuardians of the Pole
Head of CetusHeavenly GHyades
Hydra HeadJob's CoffinKeystone
KidsLozengeMilk Dipper
Northern CrossNorthern FlyPleiades
Segment of PerseusSickleSquare of Pegasus
Sword of OrionVenus MirrorY of Aquarius
source: seds

An asterism is a group of stars that appear to form a visual symbol in the night sky, but also are part of a constellation, which itself is a visual symbol in the night sky. The best known asterism is the Big Dipper, which is part of the constellation Great Bear. The stars in the Great Bear that form the Big Dipper are alpha UMa, beta UMa, gamma UMa, delta UMa, epsilon UMa, zeta UMa and eta UMa. Other well-known asterisms include the Belt of Orion, which is created by the stars delta Ori, epsilon Ori and zeta Ori. The Sword of Orion is composed of the stars theta Ori and iota Ori. The Northern Cross is formed by stars of the constellation Cygnus known as alpha Cyg, beta Cyg, gamma Cyg, delta Cyg and epsilon Cyg. The beautiful asterism Pleiades is an open star cluster located in the constellation Taurus. Pleiades also is known as the Seven Sisters and, in Latin America, the Seven Little Goats. Other asterisms are the Beehive, Bier, the Bull of Poniatowski, Circlet, Coalsack, Frederick's Glory, Guardians of the Pole, Head of Cetus, Heavenly G, Hyades, Hydra Head, Job's Coffin, Keystone, Kids, Lozenge, Milk Dipper, Northern Cross, Northern Fly, Segment of Perseus, Sickle, Square of Pegasus, Venus Mirror, and Y of Aquarius.

details > Table of Asterisms [SEDS]

Diagram of the Big Dipper

The Big Dipper

What is and isn't the Big Dipper? Native American legend describes the bowl as a bear and the handle as three warriors chasing it. In the fall when the the asterism was low in the evening sky, tree leaves turned red because hunters had injured the bear, drawing blood, according to legend.

Elsewhere on Earth, local cultures saw the Big Dipper as a plow, a cart, a wagon and even a bull's thigh. The Chinese are said to have seen it as representing the government.

In the United States, the Big Dipper played the role of a Drinking Gourd in the Civil War. The Underground Railroad wasn't a real railroad, but rather a network of people who helped slaves escape from the South. Most slaves couldn't read so word was spread by song — follow the Drinking Gourd north to a better life.

What is the Little Dipper?


The North Star is a very well known object, also seen in the sky constellation Ursa Minor. Its proper name is Polaris, and it is the star seen as nearest to Earth's North Celestial Pole. If you were to stand at Earth's north pole, Polaris would be just about directly overhead. In fact, you can always tell which way is north by finding Polaris in the sky. Using a little math, the angle of Polaris above the horizon reveals your latitude on Earth. The North Star always has been the most important star for ships navigating the global oceans.

Of course, things change. Polaris won't always be the North Star. Earth's wobbles around its axis like a spinning top. The wobble is precession brought on by the the gravitational attraction of the Sun and the Moon and the fact that Earth is not a perfect sphere. About 14,000 years from now, the star Vega will move into position as our North Star. But then, after another 14,000 years, Polaris will return to the North Star position.

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