Karen Hill

Whenever a comet makes a pass around the Sun, it loses some of its matter. The ices melt and the dust streams off in a tail. Inevitably, the comet will be reduced to a meteor or to celestial confetti, or resemble an asteroid, or a burned-out comet. Some comets collide with the Sun or a … Read more

Meteors are pieces of celestial debris of any size. They can be found just about anywhere in the solar system. Cornets are a great source of meteors because they leave particles along their paths. When Earth orbits through the cometary residue, earthlings can see the dust fall in meteor showers, also known as shooting or … Read more

Not all meteors come in showers. Under clear, dark skies, you should be able to see five or six meteors a night, if you stay up all night. During an annual shower, or a storm, you can see hundreds a night. Meteor showers occur as Earth travels through the orbital path of a specific comet. … Read more

A comet is often described as a dirty snowball. A comet’s nucleus, or solid core is, in fact, made of 50 percent dust and gases, and 50 percent ice. The dust is made up of rock and metal; the ice is actually frozen gas. Both are believed to be left over from when the solar … Read more

Our knowledge of comets is based on what we see as they travel nearest the Sun in their orbits around the star. Except for comets with relatively short orbits, we don’t know how far or precisely where else they travel. As a comet nears the Sun, the ice in the nucleus begins to melt, creating … Read more

The gas and dust in a comet’s tail are extremely lightweight. The tails are created by solar wind rushing past the orbiting comet and blowing material away from the defrosting coma. When the comet approaches the Sun, the force of the wind pushes the tail out behind the comet. When the comet is moving away … Read more

Some comets may come from the Kuiper Belt, but most probably originate in the Oort Cloud. The celestial bodies in the Kuiper Belt, however, prompt debate over the differences between planets, planetesimals, and asteroids. Astronomer Gerard Kuiper studied the satellites of the outlying planets and, in the 1940s, discovered the moons Miranda and Nereid of … Read more

Comets probably come from the Oort Cloud and the Kuiper Belt. These aren’t places in a fairy tale. The Oort Cloud, named after astronomer Jan Oort, is theoretically a loose shell of leftover matter from the solar system’s formation. Approximately 98,000 astronomical units (AUs) thick, it encircles the Sun beginning about 2,000 AUs away. The … Read more

Comets are probably ejected from the Oort Cloud or Kuiper Belt as a result of a collision. Once disturbed, they are caught by the Sun’s gravity and fly into an orbit around the star. Comets that orbit the Sun once every 200 years, or less, are called short-period comets, and probably come from the Kuiper … Read more

Solar wind refers to streaming electrically charged atomic particles that constantly escape from the Sun through coronal holes, which are weak spots in the Sun’s magnetic field. It is faster and much hotter than Earth’s wind. Solar wind is traveling at about 1 million miles, 1.6 million km an hour by the time it gets … Read more

The Sun’s corona, or outer layer of atmosphere, can only be seen during a solar eclipse. At other times, the brightness of the Sun makes the corona disappear from sight, just as the Sun’s light makes it impossible to see stars in the daytime. Astronomers artificially block the Sun’s surface with special equipment, so as … Read more

Particles jettisoned from the Sun during a solar flare or contained in the solar wind can reach Earth in a matter of hours or days. Above Earth’s North and South Poles, the planet’s magnetic field is strongest and the atmosphere is filled with particularly highly charged atoms. The interaction of solar and atmospheric particles results … Read more

What Is the Solar System and How Many Planets Are There In the Solar System? Generally, the solar system could be said to contain everything that revolves around the Sun, which lies at its center, hence its name, solar means “sun”. The solar system is made up of comets, meteors, and dust; the minor planets, … Read more

The solar system did not form all at once. We know that the Sun started shining about 5 billion years ago, but the planets came afterwards. Astronomers estimate that Earth was likely formed some 4.6 billion years ago and that the other planets, asteroids, and satellites coalesced at about the same time. It is safe … Read more

There is no definite boundary to the solar system. The center of the system is the Sun. The farthest identifiable bit of the solar system may be an icy cloud formation called the Oort Cloud from which comets appear. It extends about 2 light-years away. The nearest star to the Sun is Alpha Centauri, 4 … Read more

Sunspots are strange shadows on the Sun’s surface and vary in size and groupings. About 75 percent of sunspots come in groups of two or more, with one leading umbra, or center dark patch, that is usually the first to appear and the last to vanish. The largest recorded group of sunspots was measured on … Read more

Solar faculae are hot areas that appear on the Sun’s surface and are actually clouds of hydrogen just above the photosphere. Faculae can be easily seen when at the edge, or limb, of the Sun’s disk, but are difficult to distinguish toward the center of the star. Faculae are produced by concentrations of magnetic field … Read more

Sunspots have great magnetic forces, which fluctuate in intensity and polarity as does Earth’s magnetic field. When the magnetic field of a sunspot loses its stability, it can cause a violent explosion, called a flare. Flares look like sudden brilliant stars within a star, generally lasting less than an hour. The explosion sends shock waves … Read more

Tall, vertical streams of gas, called spicules, are like huge fiery geysers shooting up into the Sun’s chromosphere and then disappearing. Spicules are usually associated with regions of high magnetic flux. They rise about 6,000 miles, or 10,000 km and last between 5 and 15 minutes. Spicules were discovered in 1877 by Father Angelo Secchi … Read more

Solar prominences appear as flaming arches at the limb, or edge, of the Sun’s disk. Electrically charged gases in solar prominences come from the magnetic fields around sunspots. Prominences last for weeks or even months, and arc tens of thousands of miles into the Sun’s chromosphere. When a solar prominence is viewed from a different … Read more

The center of the Sun appears brighter than its edges because it is hotter. The Sun’s heat is evidence of the star’s energy. The constant energy, fueled by thermonuclear fusion, can be seen on its surface in undulating waves, called granules. But the energy is not always stable; it fluctuates in intensity. Some disturbances of … Read more

The vast distance to the Sun makes its surface look smooth from Earth, but, on the contrary, the Sun’s photosphere is bumpy. The Sun is made of turbulent hot gases, rising from the interior. The movement of the gases creates an undulating surface, which looks something like the skin of an orange. Every bump of … Read more

Sunspots seem to be visual evidence of magnetic disturbances in the Sun’s convective or radiative layers. In other words, sunspots are areas of highly intense magnetic energy, which are dark because they are much cooler than their surrounding areas. They usually occur in pairs, and sometimes in groups. The center of a sunspot, known as … Read more

There may be some connection between the two solar phenomena known as faculae and sunspots. While sunspots are cooler, darker patches of the photosphere, faculae are hotter, brighter areas. Faculae always appear a few hours or days before new sunspots develop. Sunspots at the edge, or limb, of the Sun’s disk, are accompanied by faculae. … Read more

Sometimes you see sunspots and sometimes you don’t. Individual sunspots come and go within days or weeks, the newer ones appearing closer to the equator. Sunspot activity, however, has an average cycle of 11 years. Some scientists claim that 11 years is only one-half of a full 22-year cycle. The cycle begins with the appearance … Read more

Sunspot have an irregular cycle of 11 years, though intervals as short as 7 years and as long as 17 years have been recorded. Not every cycle displays the same number or size of sunspots. In particular, between A.D. 1645 and 1715, an era dubbed the Maunder Minimum after British astronomer E. W. Maunder, who … Read more

Scientists expect that after another 5 billion years, the Sun will have used up its hydrogen energy supply and will begin to burn helium. At that point, the Sun will likely grow to become a red giant, big enough to encompass the orbit of Mars. Like other red giants of its size, its outer layer … Read more

The Sun measures 865,000 miles, or 1,392,000 km in diameter, or about 109 times that of Earth. It would take more than 1.3 million planets the size of Earth to fill the Sun. On a scale, the Sun would weigh almost 333,000 times more than Earth. Its mass is about 2 × 10^30 kilograms. The … Read more

Like other stars, the Sun is composed mostly of gases. About 70 percent of the gas is hydrogen and 25 percent is helium. Just like other stars, the Sun’s hydrogen produces energy by changing into helium through the process of thermonuclear fusion. We see the Sun’s energy in the form of sunlight and feel it … Read more

At the center of the Sun lies its core, where hydrogen fuses into helium, creating energy. The core is estimated to be about 280,000 miles (450,000 km) in diameter. The core of the Sun is the only location in the Sun that produces a significant amount of heat through fusion. From the core out, there … Read more

The Sun, a main sequence star, is at a stage of its life during which it is fueled by thermonuclear fusion. In this process, hydrogen converts to helium, creating massive amounts of energy. Just as with all the stars we see shining in the sky, some of this energy radiates as visible light, rays of … Read more

It is necessary to discuss looking directly at the Sun because every year people damage their eyes by not taking precautions, especially during eclipses. This is no joke. Looking at the Sun, even for a few seconds, with the naked eye or through binoculars or telescopes may result in blindness, permanent or temporary, or other … Read more

Astronomers don’t always have evidence of a celestial object itself, but must rely on their knowledge of the neighborhood of a suspected object, such as a black hole or neutron star. For instance, they may see a patch of sky where everything, all light, all radiation, appears to be streaming to one point. Perhaps the … Read more

Scientists hypothesize that the leftover core of a supernova would be like the entire Sun squeezed into an area 10 miles (16 km) wide. The remaining core, called a neutron star, would be made of neutrons, atoms stripped of their electrons, or electrical charges, and would lack the energy to emit light. The particles of … Read more

Black holes were first discussed in 1783, but their existence has not been proven beyond a shadow of a doubt. Astronomers theorize that if a large mass were squashed into a tiny space, the resulting power of gravity would attract and devour all nearby matter, even light. It would be a black hole. Black holes … Read more

The star nearest to us, the Sun, is a mature star. It is classified as a red dwarf. In terms of spectral class, it is a G2V star. G2 indicates its surface temperature of approximately 5,778 Kelvin. Its absolute magnitude is +4.8, so it is of average brightness for a star. Because the Sun is … Read more

One piece of evidence is lacking from astronomers’ theory of thermonuclear fusion within stars. Thermonuclear fusion, which creates solar energy, releases a great quantity of subatomic particles called neutrinos. Huge numbers of them ought to come from the Sun. After careful experimentation, however, astronomers have found only 10 percent of the neutrinos they expected. The … Read more

Eventually, the helium shells of most white dwarfs burn themselves up and their core temperatures and luminosities drop. Finally, the stars simply die out, leaving behind cold balls of carbon called black dwarfs. The material in a white dwarf no longer undergoes fusion reactions, and is supported only by electron degeneracy pressure, causing it to … Read more

The core of an old star that has lost most of its outer mass is called a white dwarf, also known as a degenerate dwarf. It is mostly carbon, with a relatively thin shell of burning helium. The core temperature remains too low to produce nuclear fusion, so the atoms simply continue to contract. The … Read more

The end of a star’s life depends on its mass. Small stars die pretty quietly, becoming planetary nebulae, white dwarfs, and then black dwarfs. Very large stars explode in supernovae, possibly turning into neutron stars or black holes. Most of the matter in the star is blown away by the supernovae explosion forming nebulae, and … Read more

A nova is nuclear explosion caused by hydrogen on the surface of a white dwarf star, which ignites and starts nuclear fusion in an unstable manner. Many stars travel in couples called binary star systems. When one of the stars becomes a white dwarf, its force of gravity sometimes attracts matter from its companion star. … Read more

A star many times more massive than the Sun ends life in a spectacular explosion called a supernova. Whereas the core of a smaller star turns into carbon too cool to create nuclear fusion, the core of a massive star becomes hot enough to spark carbon fusion. Carbon fusion turns the core into a steel-like … Read more

After a red giant has used up its helium, its outer layer is thrown off into space. The jettisoned shell, made up mostly of hydrogen, is called stellar wind. The star’s deeper layers form an expanding shell, about 0.5 to 1 light-year wide, known as a planetary nebula. Early astronomers wrongly thought that these masses … Read more

For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star’s interior which then radiates outward. Stars are mostly made up of hydrogen atoms. At 10 million°K (about 18 million°F or 10 million°C), the nuclei of hydrogen atoms fuse, or join together and change into … Read more

A star’s age or maturity is marked by its location on the main sequence of the Hertzsprung-Russell diagram. Where it first appears on the main sequence, however, depends on its mass. A star with greater mass will be hotter, take less time to mature, and will appear on the upper part of the diagram. A … Read more

All main sequence stars are in hydrostatic equilibrium, where outward thermal pressure from the hot core is balanced by the inward gravitational pressure from the overlying layers. As stars use up most of the hydrogen in their cores through thermonuclear fusion, they leave the main sequence and pass into old age. Once a main sequence … Read more

The lifespan of a star depends on how much mass the star has. Stars feed themselves by turning hydrogen into helium. Through this process, they lose mass. You could say a star consumes itself to stay alive. It spends 90 percent of its life consuming itself. Hot, bright blue giants have more hydrogen turning into … Read more

The core of a red giant or super giant, also known as an old star, is mostly helium, with an outer shell of burning hydrogen. The helium contracts while the hydrogen expands. The star grows brighter and much bigger as the hydrogen escapes into space as radiation. Expansion cools the shell, but contraction heats the … Read more

Three types of variable stars grow brighter and dimmer at regular intervals: Cepheids, RR Lyrae, and long-period variables (LPVs). These stars are probably between old age and death. Cepheids take anywhere from about 1 to 70 days to go from their brightest to their dimmest and back to their brightest. The brightness of RR Lyrae … Read more

Different stars have different surface temperatures, depending on their chemical composition, size, and age. Astronomers have developed seven basic classifications for stars according to temperature, which are called spectral classes. From hottest to coolest, the classes are 0, B, A, F, G, K, and M. Class 0 stars have surface temperatures greater than 30,000°K (53,540°F/29,727°C). … Read more

Luminosity (L) is the measure of energy, or power emitted by a star. It is measured the same way we measure the energy output of a lightbulb. Lightbulbs come in different strengths: 40 watts, 60 watts, or 100 watts, for instance. A watt is a unit of energy emission. The Sun’s luminosity, or energy output, … Read more

Two scientists, Ejnar Hertzsprung and Henry N. Russell, discovered the connection between a star’s luminosity, absolute magnitude, and temperature. They created a graph, called the Hertzsprung-Russell diagram, on which all stars can be plotted according to these three characteristics. When stars are put onto the graph, 90 percent of them fall along one diagonal path, … Read more

Stars can be said to go through the same stages of life as people: birth, infancy, maturity, old age, and death. As with people, stars are not all the same. The differences in their lives, their spectral class, life expectancy, and eventual end, depend primarily on their mass, or the amount of matter they have. … Read more

A new star, or protostar, does not yet shine. It is a loose mass of cold dust and gases. Gravity forces the protostar to contract during its infancy, which lasts for millions of years. A protostar trillions of miles wide will shrink to only millions of miles in diameter. The motion of the shrinking protostar … Read more

Space is full of clouds of dust and gases, known as nebulae. The dust and gases in nebulae have a certain mass, and we know, from Newton’s law of gravity, that any object with mass attracts other objects to it. The dust and gases are gradually pulled toward each other by the force of gravity. … Read more

The regions between superclusters, called voids, generally contain no galaxies. Voids make up approximately 95 percent of the universe. Contrary to what you may think, voids are probably far from empty. Some astronomers believe that most of the matter in the universe resides in the voids, probably in the form of dark matter. Dark matter … Read more

For a long time, scientists thought that galaxies were quiet, stable citizens of the universe. On the contrary, however, many galaxies engage in great, even violent, activity. Seyfert galaxies, for instance, emit 100 times the electromagnetic waves of ordinary galaxies. Active galaxies are known to collide and produce massive nuclear explosions, sending jet streams of … Read more

There are three ways to talk about a star’s brightness. We can discuss the apparent magnitude of a star, or how bright it appears when we look at it. We know, however, that stars nearer the earth appear brighter than faraway stars, even though they may not actually be brighter. So, astronomers also talk about … Read more

A star is a big luminous ball of plasma held together by gravity. Stars are made mostly of hydrogen and helium, along with small amounts of calcium, iron, and titanium oxide. The amounts of these elements differ from star to star. Astronomers can tell what elements are in a star, and how much of each, … Read more

Apparent magnitude, or how bright a star appears when we look at it, is always defined in comparison to another star. An increase in apparent magnitude by a factor of 1 means that a star is 2.5 times brighter than another star. The star Rigel appears brighter than the star Pollux; Rigel has an apparent … Read more

When we look at stars, the closer ones usually appear brighter than the ones farther away. Absolute magnitude is how bright each star would look if they were all 10 parsecs, or 33 light-years away. For instance, the Sun appears to be the brightest star. It has an apparent magnitude of -27.72. Altair appears about … Read more

Most astronomers believe that all of the galaxies were formed around the same time, 10 to 20 billion years ago. They probably came into existence only a few hundred million to a billion years after the big bang. The nucleus of a galaxy is a powerful source of energy. It is commonly thought that some … Read more

Although some stars travel alone in galaxies, most are grouped in star clusters. There are two types of star clusters, open clusters and globular clusters. Open clusters contain between 10 and 10,000 stars loosely distributed over 15 to 40 light-years. Most of the stars are relatively young, bright, and hot. Open clusters appear within the … Read more

There are many theories about galaxy formation. Most of the theories contain the basic notion that there must have been gravitational seeds, regions that had more mass, and, hence, more gravity, than everything around them. Those seeds evolved into the galaxies of today. Early discussions of the big bang theory included the idea that all … Read more

A quasar is very energetic and very distant galaxy with an active galactic nucleus, and are the most luminous objects in the universe. Because, in space, distance equals age, they are also the youngest objects. Quasar stands for “quasi-stellar radio source”. Discovered in 1960, quasars seem to be very small, about the size of our … Read more

Most galaxies in the universe are grouped together in clusters because of their mutual gravitational attraction. These clusters contain anywhere from just a few to thousands of galaxies. Within clusters, galaxies orbit each other at speeds of about 600 miles per second. In turn, galactic clusters often come in groups called superclusters, which can be … Read more

Interstellar medium is the term for the gas and dust that exists between stars. Gas makes up 99 percent of interstellar medium; dust, only 1 percent. Radio waves have shown that most of the gas is made of hydrogen (75 percent) and helium (23 percent), with smaller amounts (2 percent) of carbon monoxide, carbon, oxygen, … Read more

The Local Group is a cluster of about thirty galaxies which includes the Milky Way. Even with the naked eye, we can see beyond our galaxy to others in the Local Group. Our twin galaxy Andromeda is visible even though it is more than 2 million light-years away. The Milky Way and the Andromeda Galaxy … Read more

The nuclear bulge of the Milky Way cannot be seen clearly from Earth. Using the Andromeda Galaxy as a visual guide to our own, we believe that the nuclear bulge is a collection of star clusters and clouds of dust and gas. The detection of gamma ray, X ray, infrared, and radio waves gives us … Read more

There are three kinds of nebulae, based on the light they reflect. Emission nebulae glow greenish-yellow and red. This is caused by the nearby presence of one or more extremely hot stars. The ultraviolet radiation from the stars causes the hydrogen and oxygen of the nebulae to glow. Reflection nebulae contain one or more cooler … Read more

Nebulae is the Latin word for clouds, nebula is the singular form. Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium. As the matter collapses under its own weight, huge stars may form in the center, and their ultraviolet radiation ionises the surrounding gas, making it visible to the … Read more

The Milky Way is believed to be moving in the direction of the Great Attractor, which is a gravity anomaly in intergalactic space within the range of the Centaurus Supercluster. Astronomers also believe the Milky Way is moving at about 630 km per second, although we must keep in mind the absolute velocity of any … Read more

Although all galaxies are composed of the same things, each one is unique. Some galaxies are mostly made up of clouds of dust and gases. Others contain more stars than clouds. Galaxies can appear flat or spherical. Some are bright and clear; others are dim and obscure. Galaxies can stand alone or interact with other … Read more

Spiral galaxies were originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”. Spiral galaxies, such as our own Milky Way, look something like pinwheels. They have a central, circular, cloudy-looking mass of stars with arms spiraling out from the center. Barred spiral galaxies have bar-shaped centers with arms coming off … Read more

Elliptical galaxies in our universe have the shapes of ellipses, or stretched-out circles. They can be egg-shaped, almost round, or almost flat. The astronomer Edwin Hubble created a scale from 0 to 7 to designate the roundness of an elliptical galaxy. An elliptical galaxy that is almost round is called E0, while one that is … Read more

We live in the Milky Way Galaxy. Sometimes, on dark, clear nights, you can see part of the Milky Way as a cloudy, or milky, river of stars in the night sky. Our solar system happens to be on the edge of the galaxy, the Sun is about 30,000 light-years from the center, so we … Read more

Galaxies that do not share a common shape fall into the catchall category of irregular galaxies. They are populated mostly by young stars, though old ones also appear, as well as gas and dust. Irregular galaxies are uncommon, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral … Read more

The Milky Way is a spiral Galaxy that is estimated to be between 10 and 20 billion years old. It is about 100,000 light-years at its widest and 15,000 light-years at its thickest in the center. It contains more than 100 billion stars, maybe even 200 billion, most of which are concentrated in the nucleus … Read more

At least 90 percent, or the bulk of matter in the universe, making up its mass, is a mystery. We don’t know what it is, and not all scientists believe that dark matter exists. All matter we know about is made up of particles called photons and neutrons. Photons and neutrons are the building blocks … Read more

Some scientists theorize that the amount of matter is not the most important factor in the fate of the universe. They explain that a cosmological constant will keep the universe forever balanced between infinite expansion and inevitable collapse. One view of the cosmological constant, known as the steady state theory, holds that new matter is … Read more

Since the force of gravity relates precisely to the mass of an object, we can estimate the mass of the universe by observing the forces of gravity in the universe. We also can add up the known mass of different atoms and particles in the universe. The problem is that when astronomers add up the … Read more

A galaxy is a loose formation of between 1 million and 1 trillion stars, dust, and gases. These stars, dust, and gases are drawn together in clusters by their forces of gravity, much in the same way as our solar system is held together by the gravity of the Sun and the planets. It is … Read more

Galaxies are the largest, most prominent structures in the universe. However, our telescopes may not be powerful enough to count all of them. With data from the Hubble Space Telescope, a telescope high above Earth’s atmosphere, scientists recently revised their estimate of the number of galaxies from mere billions to at least hundreds of billions. … Read more

By studying galactic light that originates 300 million light-years away, astronomers can look at galaxies that are 300 million years younger than the Milky Way. The light we see today started traveling 300 million years ago, so it represents the source galaxy as it was 300 million years ago. Galaxies at different distances portray different … Read more

In the immediate aftermath of the initial rapid “vavoom”, or big bang that started it all, very little matter existed. Scientists say the big bang was not an explosion, but a burst or surge. Immediately after, subatomic particles combined to create protons and neutrons. Protons and neutrons form the nucleus of atoms. The first atoms … Read more

Current theories say the universe’s continued expansion depends on how much physical matter exists in the universe, and we aren’t sure just how much stuff there is. The amount of matter that would keep the universe stable is called omega. Omega is a hypothetical, unknown quantity of matter, but scientists have determined what is likely … Read more

Unfortunately, we can’t get outside the universe to see what it looks like as it expands, but we can imagine it. If you took a balloon and marked it with dots and then blew up the balloon, the dots would expand along with the balloon’s surface. As the balloon grew larger, the dots would move … Read more

Try this exercise to visualize the universe as astronomers see it. Take a deflated balloon and draw small stars on it with a marker. The stars represent galaxies. Label one the Milky Way, for our galaxy. Now, blow up the balloon. The growing balloon is similar to the expanding universe. You will see how the … Read more

We don’t know for sure what the end of the universe will be like, or whether there will even be an end. The big crunch theory refers to gravity causing everything in the universe to contract and collapse in upon itself. There are two results that scientists generally propose. Either matter will collapse into a … Read more

A black hole is a fascinating hypothetical concept and does not refer to a hole in the proper sense of the word. A black hole is an object, a massive amount of energy in a tiny amount of space. Very large stars, much more massive than the Sun, for instance, will likely end their existence … Read more