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The Galaxy

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This beautiful galaxy is tilted at an oblique angle on to our line of sight, giving a 'birds-eye view' of the spiral structure.    The perfectly picturesque spiral galaxy known as Messier 81, or M81, looks sharp in this new composite from NASA's Spitzer and Hubble space telescopes and NASA's Galaxy Evolution Explorer. M81 is a "grand design" spiral galaxy, which means its elegant arms curl all the way down into its center. It is located about 12 million light-years away in the Ursa Major constellation and is one of the brightest galaxies that can be seen from Earth through telescopes.
The colors in this picture represent a trio of light wavelengths: blue is ultraviolet light captured by the Galaxy Evolution Explorer; yellowish white is visible light seen by Hubble; and red is infrared light detected by Spitzer. The blue areas show the hottest, youngest stars, while the reddish-pink denotes lanes of dust that line the spiral arms. The orange center is made up of older stars.

NASA, NOAA Ready GOES-P Satellite for March 2 Launch

NASA is preparing to launch the NOAA Geostationary Operational Environmental Satellite-P (GOES-P) on March 2 during a launch window from 6:19 to 7:19 p.m. EST from the Cape Canaveral Air Force Station, Fla. › Learn More


NASA's Space Shuttle Program Successfully Conducts Final Motor Test in Utah

The flight support motor, or FSM-17, burned for approximately 123 seconds -- the same time each reusable solid rocket motor burns during an actual space shuttle launch. › Learn More

Former NASA Ames Scientist Wants Energy to 'Bloom' Throughout the World

K.R. Sridhar used to spend his time as a researcher at NASA Ames Research Center looking at the sky and dreaming of ways to sustain life on Mars. Now, he is CEO of Bloom Energy. › Learn More

Print these free astronomy and space posters from your home computer to create your own astronomy and space posters. Supernova Blast Bonanza in Nearby Galaxy The nearby dwarf galaxy NGC 1569 is a hotbed of vigorous star birth activity which blows huge bubbles that riddle the main body of the galaxy. The galaxy's "star factories" are also manufacturing brilliant blue star clusters. This galaxy had a sudden onset of star birth about 25 million years ago, which subsided about the time the very earliest human ancestors appeared on Earth.
Print these free astronomy and space posters from your home computer to create your own astronomy and space posters.

Space Phenomenon Imitates Art in Universe's Version of van Gogh Painting This image resembling Vincent van Gogh's painting, "Starry Night," is Hubble's latest view of an expanding halo of light around a distant star, named V838 Monocerotis (V838 Mon). This Hubble image was obtained with the Advanced Camera for Surveys on February 8, 2004. The illumination of interstellar dust comes from the red supergiant star at the middle of the image, which gave off a flashbulb-like pulse of light two years ago. V838 Mon is located about 20,000 light-years away from Earth in the direction of the constellation Monoceros, placing the star at the outer edge of our Milky Way galaxy.
Print these free astronomy and space posters from your home computer to create your own astronomy and space posters.

Too Close for Comfort
This Hubble Space Telescope view of the core of one of the nearest globular star clusters, called NGC 6397, resembles a treasure chest of glittering jewels. The cluster is located 8,200 light-years away in the constellation Ara. Here, the stars are jam-packed together. The stellar density is about a million times greater than in our Sun's stellar neighborhood. The stars in NGC 6397 are also in constant motion, like a swarm of angry bees. The ancient stars are so crowded together that a few of them inevitably collide with each other once in a while. Near misses are even more common.
Print these free astronomy and space posters from your home computer to create your own astronomy and space posters. Ghost Nebula This ghostly apparition is actually an interstellar cloud caught in the process of destruction by strong radiation from a nearby hot star. This haunting picture, snapped by the Hubble telescope, shows a cloud illuminated by light from the bright star Merope. Located in the Pleiades star cluster, the cloud is called IC 349 or Barnard's Merope Nebula.
Print these free astronomy and space posters from your home computer to create your own astronomy and space posters. Lunar Footprint Buzz made this footprint on a pristine surface so that he could then photograph it for study by soil mechanics experts.
Print these free astronomy and space posters from your home computer to create your own astronomy and space posters.

Montage This montage of images taken by the Voyager spacecraft of the planets and four of Jupiter's moons is set against a false-color Rosette Nebula with Earth's moon in the foreground. Studying and mapping Jupiter, Saturn, Uranus, Neptune, and many of their moons, Voyager provided scientists with better images and data than they had ever had before or expected from the program. Although launched sixteen days after Voyager 2, Voyager 1's trajectory was a faster path, arriving at Jupiter in March 1979. Voyager 2 arrived about four months later in July 1979. Both spacecraft were then directed to Saturn with Voyager 1 arriving in November 1980 and Voyager 2 in August 1981. Voyager 2 was then diverted to the remaining gas giants, Uranus in January 1986 and Neptune in August 1989. Data collection continues by both Voyager 1 and 2 as the renamed Voyager Interstellar Mission searches for the edge of the solar wind influence the heliopause and exits the Solar System
  The Galaxy: general

The position of the sun in our Galaxy, the Milky Way, gives us an opportunity to study the evolution of the stars in detail and to investigate the formation and evolution of galaxies. The Milky Way is a typical spiral galaxy. It has recently been discovered that it is also barred in the middle, as seen in other spiral galaxies and for example in the galaxy M100 (below). The solar system is located in the plane near the edge of the disk, therefore we see the disk edge on. With the naked eye the disk of our Galaxy appears as a milky band made up of a myriad of stars and interstellar clouds.

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M100 galaxy. Photographed by Hubble Space Telescope (Credit NASA).
This spiral galaxy, similar to our Milky Way, is to approximately 60 million light-years, in the Virgo Cluster. In the spiral arms, stars are born. They are very hot and give a blue color. In the central part of the galaxy, the white light comes from stars hurdy-gurdies.

This spiral galaxy, similar to our own Galaxy, is 60 million light-years from us, in the Virgo galaxy cluster. The spiral arms are filled with interstellar clouds of gas and dust, and young stars. At birth these massive stars are hot and blue, which gives the observed blue colour to the arms. In the central part of the Galaxy, stars were born earlier in the history of the galaxy. Hence they are are now older, redder and cooler than the blue stars. Many different kinds of observation can be made on stars using modern telescopes.

We actually measure their luminosity, colour in various part of the spectrum, temperature, gravity, abundance of various elements, motions and sometimes also their age and mass. With this data for a large number of stars one can try to deduce how the Galaxy has been forming stars, how old it is as well as how it has evolved. Finally we emphasise an overall scheme of formation and evolution for all galaxies. One way to further our understanding of Galactic structure and evolution is to make a mathematical model.

We have used this method and so have built a Galaxy model which allows us to simulate the number of stars and their observational properties expected in any given region of the sky, by using a set of theoretical assumptions and a scenario of Galactic evolution. The Besançon model of the Galaxy is a powerful tool dedicated to the understanding of how the properties and statistics of the stars are related to the history of our Galaxy.
Additional Links
Galaxy Zoo  NASA's Galaxy Facts  Galaxies from World Book 
Galaxy Evolution Explorer  Chandra's Latest News  Star Child  An Atlas of the Universe  Dynamical Astronomy Java Lab
Lightning at Weikerscheim Observatory

Astronomy only plays a tangential role in this picture… you can see stars in the sky, and the observatory is pretty obvious. But it’s the terrestrial drama that steals the show.

Jens Hackmann took this stunning picture of a lightning storm near the Weikerscheim Observatory; the 300 second exposure is enough to see the stars streak and the observatory lit up by ambient light. Sometimes, when it’s cloudy, observing is difficult… but you can still get incredible pictures.

A Meeting of the Moons

Jupiter is fantastically massive, and its fearsome gravity holds thrall over a retinue of moons that might otherwise be called planets in their on right. By far, the most interesting of its family are the moons Europa and Io. Europa is an ice world, covered in a thick sheet of ice that might reach down to depths of over perhaps several kilometers. It’s nearly a dead certainty that underneath that forbidding icecap is an ocean of water, kept liquid from energy input by gravitational stress as Europa passes by her sister moons. Of all the real estate in the solar system, many astronomers have their money on Europa as the best place to look for alien life.

Io, on the other hand, is perhaps the worst place for life. It has an incredibly high sulfur content, for one thing. For another, the same gravitational heating that keeps Europa’s ocean liquid also keeps Io’s interior molten, but it gives the moon a cosmic case of indigestion.

Io is wracked with volcanoes. They are almost constantly erupting, spewing molten sulfur over a kilometer high in the low gravity, and plumes of dust and gas blast hundreds of kilometers off the surface. This activity was first discovered when the Voyager 1 probe passed the moon in 1979, but subsequent space probes have gotten even more detailed images. When the New Horizons Pluto probe passed Jupiter in March 2007 for a gravity boost, it snapped a beautiful picture of the sisters.


Europa is the crescent on the lower left, and obviously is the one on the upper right. The plume you see is from the volcano Tvashtar, which has been active for quite some time now. If you look right at the bottom of the plume, you can see molten sulfur glowing red. Two other volcanoes appear to be making some noise as well.

While they appear to be close together, the two moons were actually nearly 800,000 kilometers apart when this picture was taken; Io was on one side of Jupiter and Europa on the other, but from the spacecraft’s perspective they were next to each other in the sky. This picture is actually a composite of two images; one was greyscale and had high resolution, and the other was in color but had lower resolution. By merging the two, we can see more details than we could from the color image alone, and we get the benefit of having the colors enhance the scene.

When I first saw this image, I knew right away the two moons were not close together at all. My secret? I saw that the dark side of Europa was truly dark, but Io’s dark side was lighter. That meant that Io was positioned such that Jupiter was illuminating its otherwise dark half, while Europa must have been on the other side of Jupiter, where it was dark. Sometimes, you can tell a lot just by looking at a image and picturing the geometry in your head.

I have long been a fan of Davide De Martin from Sky Factory. He takes images from professional observatories and stitches them together to make images of indescribable beauty, elegance, and wonder. His work is, simply, breathtaking.

This image shows the devastation wrought when a star explodes. The Vela Supernova Remnant formed when a massive star 800 light years away blew up 11,000 years ago. Expanding at a ferocious velocity, it is now 8 degrees across in the sky — 16 times the apparent width of the Moon, and about the size of your outstretched fist! David’s mosaic shows a stunning amount of detail, tracing the variety of shapes and patterns the expanding gas makes as it slams into the interstellar junk floating around it.

And if that’s not enough, the full-size image he has on his site is well over a billion pixels in size. Think about that the next time you brag about your digital camera.Let’s start by asking a simple question: how many stars are there in our galaxy? We know the answer is roughly 400 billion stars, even though there may be, realistically, as few as half or as many as double this number. What’s interesting is that about 90% of all of the stars in the Milky Way are relatively cool and small, like our Sun (types F, G, K, and M below). Most are a little smaller (something like 80% are only a fraction of the Sun’s mass), but there are plenty of stars that are very comparable to the Sun in mass, brightness, and composition.


This means that of the 400 billion stars in our galaxy, about 10% of them are similar enough to our Sun that they could reasonably have planets around them that support life. This means our galaxy has about 40 billion chances to have another planet like Earth in it.

So the next step, now that we know how many good stars there are, is to ask how many of them have planets that could support life on them? It turns out, as far as we can tell, that most stars actually do have planets orbiting them. Although we’ve only discovered about 300 planets orbiting other stars at last count, the results are consistent with the majority of stars having planets orbiting them.

Due to limitations in the technology we currently have, we’ve only been able to find gas giant planets around other inhabitable stars, but when I say we’ve been able to find them, we’ve actually been able to optically image them. In fact, here’s the very first one, which weighs in at 8 times the mass of Jupiter:



But we have found rocky planets, and we have found planets around inhabitable stars, and they all seem abundant. The question is how many planets, in each Solar System, will be within what we call the habitable zone, or in the region around the star where they won’t freeze or be cooked? Well, our Solar System had, at its outset, three chances: Venus, Earth, and Mars. Currently, Earth is the only one with life, but Venus and Mars may both have had it in the past.


You can read more about habitable zones here, but the overall lesson is that it’s very easy to have a rocky planet in the area around a star where, at some point, you can have life, such as Mars or Venus. It’s more difficult to have a planet like Earth, where the temperatures have been just right to allow life to form and evolve for a continuous period of over 4 billion years.
  As far as I can tell, to have a rocky planet that falls somewhere in the habitable zone at some point happens frequently: maybe 5% of the time according to simulations. On the other hand, to have something Earth-like, which is the right mass to have a living core and hence be volcanically active, has a magnetic field, has a thick (but not too thick) atmosphere, and is in the continuously habitable zone, happens far less frequently. I don’t know the geophysics to predict volcanic activity, rotation speed, and magnetic fields, but I can predict the rest: we get a planet of the right mass and right atmosphere in the perfect “goldilocks” location maybe 0.01% of the time.

Still, this gives us a lot of chances. Out of the 40 billion roughly Sun-like stars, this means about 2 billion have rocky planets that could support life at one time, and there are about 4 million planets in the galaxy with favorable masses and locations as compared to Earth. If we want to be conservative, we can say that maybe only 1 in 4 of those has the right magnetic field and rotation to allow life to evolve, so that leaves us with 1 million good-looking planets for continuous life in the Galaxy.
  But then what? Just because all the elements are in place doesn’t mean we’re going to get life, does it? Unfortunately, this is the part where I have to make like OJ’s judge, Jackie Glass, and say “I don’t know.”

The Milky Way Galaxy - Our Home
The Milky Way galaxy is the spiral galaxy we call home, as do roughly 100 billion other stars. It looks very much like other spiral galaxies when viewed from above. There are spiral arms and a bright central part. The Sun is far from the center of the Galaxy, halfway to the edge of the Galaxy along the Orion spiral arm.

The Sun is revolving around the center of the Galaxy at a speed of half a million miles per hour, yet it will still take 200 million years for it to go around once. Do you feel like you are moving at that speed through space? If you did, you would certainly need a seat belt! When we run, we feel the wind on our bodies because there are molecules which make up the air that push against our bodies. But there are very few molecules in the space between the stars. So there is nothing to push against our planet so that we "feel" like we are rushing around at half a million miles per hour.


Like other spiral galaxies, the Milky Way has a bulge, a disk, and a halo. Although all are parts of the same galaxy, each contains different objects. The halo and central bulge contain old stars and the disk is filled with gas, dust, and young stars. Our Sun is itself a fairly young star at only 5 billion years old. The Milky Way galaxy is at least 5 billion years older than that.

Not long ago, many people thought that comets were a sign that something bad was about to happen to them. People didn't understand how objects in the sky moved, so the sight of a comet must have been very disturbing. There are many historical records and works of art which record the appearance of comets and link them with terrible events such as wars or plagues.

Now we know that comets are lumps of ice and dust that periodically come into the center of the solar system from somewhere in its outer reaches, and that some comets make repeated trips. When comets get close enough to the Sun, heat makes them start to evaporate. Jets of gas and dust form long tails that we can see from Earth. These tails can sometimes be millions of miles long.

In 1985-1986, a spacecraft called Giotto visited the most famous comet, Halley, on Halley's most recent visit to the inner solar system. In 1994, comet Shoemaker-Levy became trapped by the gravity of Jupiter and plunged into Jupiter's atmosphere!

In 1996 and 1997 we saw comet Hyakutake, and comet Hale-Bopp. Hale-Bopp was one of the brightest comets ever seen from Earth. Comet Linear was discovered in 1999 and made its closest approach of the Sun in July 2000. The Stardust spacecraft flew by Comet Wild 2 in January 2004, collecting samples of the comet to return to Earth. The newest comet mission is Rosetta  it will land on a comet named Churyumov-Gerasimenko!Now scientists have identified a class of comets known as small comets though they originally were just called snowballs from space!.

            Distance  Radius    Mass
Planet      (000 km)   (km)     (kg)   Discoverer   Date
---------  ---------  ------  -------  ----------  -----
Mercury       57,910    2439  3.30e23
Venus        108,200    6052  4.87e24
Earth        149,600    6378  5.98e24
Mars         227,940    3397  6.42e23
Jupiter      778,330   71492  1.90e27
Saturn     1,426,940   60268  5.69e26
Uranus     2,870,990   25559  8.69e25   Herschel    1781
Neptune    4,497,070   24764  1.02e26   Galle       1846
Pluto      5,913,520    1160  1.31e22   Tombaugh    1930

Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy.

diameter:    1,390,000 km.
        mass:        1.989e30kgtemperature: 5800 K (surface)
                     15,600,000 K (core)

The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest).

It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun.
The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol.

The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core.
The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body.

Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water.
The Sun's power about 386 billion billion megaWatts is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation.

The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K ,they look dark only by comparison with the surrounding regions. Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

A small region known as the chromosphere lies above the photosphere.

 The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse left. Temperatures in the corona are over 1,000,000 K.

It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey.

The Sun's magnetic field is very strong by terrestrial standards and very complicated. Its magnetosphere also known as the heliosphere extends well beyond Pluto.

In addition to heat and light, the Sun also emits a low density stream of charged particles mostly electrons and protons known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis.

Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed.

Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth.The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft.

Spectacular loops and prominences are often visible on the Sun's limb (left).

The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%.

The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).

The Sun's satellites

There are eight planets and a large number of smaller objects orbiting the Sun. (Exactly which bodies should be classified as planets and which as "smaller objects" has been the source of some controversy, but in the end it is really only a matter of definition. Pluto is no longer officially a planet but we'll keep it here for history's sake.)





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