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Facts About the Milky Way How Big is the Solar System? by Fraser Cain (Comments Off) by Nicholos Wethington Here are ten interesting facts about your home – well, at least your home galaxy – the Milky Way. 1. It's warped. The Milky Way is a disk about 120,000 light years across (see the Guide to Space article on the diameter of the Milky Way for more), with a central bulge that has a diameter of 12,000 light years. The disk is far from perfectly flat though, as can be seen in the picture below. What warped it? Two of the galaxy's neighbors – the Large and Small Magellanic clouds – have been pulling on the dark matter in the Milky Way like in a game of galactic tug-of-war. The tugging sets up a sort of oscillating frequency that pulls on the hydrogen gas (of which the Milky Way has lots of). Here's a more indepth article on Universe Today about How the Milky Way got its Warp. How big is the Solar System? It all depends on your definition. Some scientists think that the furthest influence of the Solar System extends out to 125,000 astronomical units (1 AU is the average distance from the Earth to the Sun), or 2 light years. Since the nearest star is 4.22 light-years away, the Solar System size could extend almost half way to the nearest star. Astronomers think that the Sun's gravitational field dominates the gravitational forces of the other stars in the Solar System out to this distance. Objects in this region will fall towards the Sun, and not the other stars. But is there anything actually out there? It's so far away that scientists have no way of detecting any objects. Astronomers believe that the Oort Cloud – the source of long period comets – extends out to a distance of 50,000 AU, and maybe even 100,000 AU. Once again, though, the Oort Cloud has never been seen directly. We just guess that it exists because comets with extremely long orbits sometimes pass near the Sun and then head back out again. The Oort cloud could have a trillion icy objects inside it. Another boundary that separates the Solar System from the rest of the galaxy is the heliosphere. This is the region where the solar wind collides with the interstellar medium and slows down. This region extends out to about 95 AU, or 3 times the orbit of Pluto. No spacecraft have ever reached the heliopause, but NASA's Voyager spacecraft are expected to cross the region within the next decade. 2. It has a halo, but you can't directly see it. The Milky Way has a halo of dark matter that makes up over 90% of its mass. Yes, 90%. That means that all of what we can see (with the naked eye or telescopes) makes up less than 10% of the mass of the Milky Way. Now, it doesn't have a halo like those old cartoon characters that die, sprout wings and play a harp in the clouds. The halo is actually invisible, though we know it exists by running simulations of what the Milky Way would look like and how fast stars inside the galaxy's disk orbit the center. The heavier it is, the faster they should be orbiting. If you assume that the galaxy is made up only of matter that we can see, then you get a rotation rate for the stars that is well below what it should be, so the rest is made up of what is elusively called "dark matter," or matter that only interacts gravitationally (so far as we know) with "normal matter". To see some images of the probable dark matter density distribution in our galaxy, check out The Via Lactea Project. 3. It has over 200 billion stars As galaxies go, the Milky Way is a middleweight. The largest galaxy known, IC 1101, has over 100 trillion stars, and other large galaxies can have more than a trillion stars. Smaller galaxies like the aforementioned Large Magellanic Cloud, have about 10 billion stars. The Milky Way has between 200-400 billion stars, but when you look up into the night sky the most you can see from any one point on the Earth is about 2,500. We aren't stuck with this many stars forever, though, because the Milky Way is constantly losing stars – through supernovae – and producing stars, netting about seven stars per year. The Milky Way wasn't always as it is today, a beautiful barred spiral. It became its current size and shape by eating up other galaxies. It's still doing so today – the Canis Major Dwarf Galaxy is the closest galaxy to the Milky Way because its stars are currently being added to the Milky Way's disk, and our galaxy has consumed others in its long history, such as the Sagittarius Dwarf Galaxy. 6. Every picture you've seen of the Milky Way from above is either another galaxy or an artist's interpretation. We can't take a picture of the Milky Way from above (yet) because we are inside the galactic disk, about 26,000 light years from the galactic center. This means that any pretty pictures you see of a spiral galaxy with elegant arms that is supposedly the Milky Way is either a picture of another spiral galaxy, or the rendering of a talented artist. Imaging the Milky Way from above is a long, long way off; however, this doesn't mean that we can't take breathtaking images of the Milky Way from our vantage point! 7. There is a black hole at the center. 4. It's really dusty and gassy. You may not think so by looking at it, but the Milky Way is full of dust and gas. And when I say full of dust, I mean that we can only see out about 6,000 light years into the disk of our own galaxy in the visible spectrum, and the galaxy is about 100,000 light years across! The dust and gas makes up a whopping 10-15% of the "normal matter" in the galaxy, with the remainder being stars. The thickness of the dust deflects visible light, as is explained here, but infrared light can pass through the dust, which makes infrared telescopes like the Spitzer Space Telescope extremely valuable tools in mapping and studying the galaxy. Spitzer can peer through the dust to give us extraordinarily clear views of what is going on at the heart of the galaxy and in star-forming regions. 5. It's made up of other galaxies. Most galaxies have a supermassive black hole at the center. Ours is no exception. The center of our galaxy is called Sagittarius A* (pronounced "A-star"), and it houses a black hole with a mass of 40,000 Suns that is 14 million miles across (about the size of Mercury's orbit). But this is just the black hole itself. All of the mass trying to get into the black hole – called the accretion disk – forms a disk that has a mass of 4 million Suns, and would fit inside the orbit of the Earth. Though like other black holes, Sgr A* tries to consume anything that happens to be nearby, star formation has been detected near this black hole behemoth. 8. It's almost as old as the Universe itself. The most current estimate for the age of the Universe is about 13.7 billion years. Our Milky Way has been around for about 13.6 billion of those years, give or take 800 million years. The oldest stars in our the Milky Way are found in globular clusters, and the age of the galaxy is determined by taking the age of these stars, and then extrapolating the age of what preceded them. Though some of the constituents of the Milky Way have been around for a long time, the disk and bulge themselves didn't form until about 10-12 billion years ago, and the bulge may have formed earlier than the rest of the galaxy. 9. It's part of the Virgo Supercluster, a grouping of galaxies within 150 million light years. As big as it is, the Milky Way is part of an even bigger structure called a supercluster. Superclusters are groupings of galaxies on very large scales (100s of millions of light years). In between these superclusters are large voids of space where any space traveler would encounter very little in the way of galaxies or matter. Our close neighbors include the Large and Small Magellanic Clouds, and the Andromeda Galaxy (the closest spiral galaxy to the Milky Way), and along with about 30 other galaxies this group of galaxies makes up what is called the Local Group. But as you get further on out, on the scale of hundreds of millions of light years, the Milky Way can be seen to be just a small part of a large grouping of galaxies 150 million light years in diameter called the Virgo Supercluster. 10. It's on the move The Milky Way, along with everything else, is moving through space, and puts to shame anything from everyday life that one could compare its speed to. The Earth moves around the Sun, the Sun around the Milky Way, and the Milky Way is part of the Local Group, which is moving relative to the Cosmic Microwave Background radiation – the radiation left over from the Big Bang, which is a convenient reference point to use when determining the velocity of things in the Universe. The Local Group is calculated to move relative to the CMB at about 600 km/s (2,200,000 km/h), which is pretty darn fast! Astronomical Units by Jerry Coffey even arbitrary unit; however, the use of AU to refer to the astronomical unit is generally accepted. The astronomical constant whose value is one astronomical unit is referred to as unit distance and given the symbol A. Originally, the astronomical unit was defined as the length of the semi major axis of the Earth's elliptical orbit around the Sun. In 1976, the International Astronomical Union revised the definition of the astronomical unit for greater precision. The new definition is the length for which the Gaussian gravitational constant takes the value 0.01720209895 when the units of measurement are the astronomical units of length, mass and time. An equivalent definition is the radius of an unperturbed circular Newtonian orbit about the Sun of a particle having infinitesimal mass, moving with a mean motion of 0.01720209895 radians per day, or that length for which the heliocentric gravitational constant is equal to 0.017202098952 AU3/d2. It is approximately equal to the mean Earth– Sun distance. Very precise measurements of the relative positions of the inner planets can be made by radar and by telemetry from space probes. As with all radar measurements, these rely on measuring the time taken for light to be reflected from an object. These measured positions are then compared with those calculated by the laws of celestial mechanics: the calculated positions are often referred to as an ephemeris and are usually calculated in astronomical units. The International Astronomical Union (IAU) currently accepted best estimate of the value of the astronomical unit in meters is 149597870700(3) m, based on a comparison of JPL and IAA-RAS ephemerides. According to the ancient astronomer Eusebius, Eratosthenes found the distance to the sun to be either as 4,080,000 stadia, or as 804,000,000 stadia. Using the Greek stadium of 185 to 190 meters, the former translation comes to a far too low 755,000 km whereas the second translation comes to 148.7 to 152.8 million km accurate within 2%. In modern use astronomical units are used to make the numbers involved in astronomy easier to use. Take for instance the distance to Mars: it is 227,939,000 km from the Sun or 1.523 AU. Astronomical units are a measurement that is equal to the mean distance between the Earth and the Sun. The International Astronomical Union currently estimates that distance to be 150 million km. It is abbreviated AU, au, or ua. In general, capital letters are only used for the symbols of units which are named after individual scientists, while au or a.u. can also mean atomic unit or In astronomy, an astronomical unit is defined as the average distance from the Sun to the Earth, or about 150 million kilometers (93 million miles). You can abbreviate astronomical unit as AU. Since the distances in astronomy are so vast, astronomers use this measurement to bring the size of numbers down. For example, Earth is 1 AU from the Sun, and Mars is 1.523 AU. That's much easier than saying that Mars is 227,939,000 km away from the Sun. Using astronomical units is even more helpful when the distances get larger. Pluto is 39.48 AU from the Sun, and the newly discovered Eris is 67.67 AU. The Oort Cloud, the source of long-period comets in the Solar System, is thought to be 50,000 AU from the Sun. Here are some handy conversions for you: 1 astronomical unit = 149,598,000 kilometers 1 astronomical unit = 92,955,887 miles 1 light year = 63,239 astronomical units 1 parsec = 206,264 astronomical units We have written many articles about measuring distance in space. Here's an article about the distance to Andromeda, and here's an article about the distance from Earth to Mars. Parallax by Tega Jessa measure the exact distance of stars. So how did they find out? They did so using geometry and you can thank the ancient philosopher Pythagoras for coming up with the concept of using geometry to determine large distances. If you remember from high school and middle school math, Pythagoras came up with a theorem that you could use any two sides of a triangle or one of its sides and one of its angles to determine the exact length of the other sides. This is how Pythagoras calculated the circumference of the Earth. The same principal was used to find the distance of a star. First off it had to be close enough to Earth to see an actual parallax. The further an object is the less it seems to move when you change your point of view. By using double the radius of the Earth's orbit as one side of a triangle and the angle of the chosen star's parallax, you could find out the exact distance to the star. This is because both unknown sides of the cosmic triangle would be the same, since the true distance did not change, only the perspective. The formula boils down to distance to the star equals the radius of Earth's orbit divided by the tangent of half the angle of a stars parallax. This method works will for stars under 100 light years away. This is due to the reason stated before that the parallax of an object diminishes as it is placed further away. The other reason is that the angle of parallax for stars is often very small. Parsec by Stephanie Bender So what is parallax? The best explanation is that it is the illusion of an object moving when you look at it from two different points of view. A perfect example is to hold up your thumb and place it in front of other objects for a frame of reference. Now try to looking at your thumb with only your left eye. Now do the same thing with your right eye. You will notice that it looked like your thumb moved in reference to the other objects behind it. Yet the entire time you know that you did not move your thumb. This visual principal also works in astronomy. If you look at certain stars they seem to have different positions in the night sky in relation to other stars depending on the time of year. This is a stellar parallax. For the longest time astronomers did not have the tools to physically A parsec is equivalent to 3.26 light years. And since a light year is the distance light travels in 1 year 9.4 trillion km, 1 parsec equals 30.8 trillion km. So, where then, does the term, "parsec", come from? Parsec is a combination of 2 words, parallax (par) and arcsecond (sec). Parallax means something looks like it changed its location because you changed yours. For example, if you stand on your porch and look across the street, you will see a house on your left and a house on your right. If you go across the street and look at the same houses from your neighbor's backyard, they will be on the opposite sides. Did the houses move? Of course not. You changed your location. Since you are in a different place, facing a different direction, they appear to be in different places. Likewise, two different people, in two different parts of the world, might see the exact same event in the sky or outer space; yet, it might appear entirely different due to their locations. Astronomers measure parallax by measuring how distant stars shift back and forth as the Earth travels around the Sun. Astronomers measure the position of the stars at one time of the year, when the Earth is at a position in its orbit around the Sun, and then they measure again 6 months later when the Earth is on the other side of its orbit. Nearby stars will have shifted a tiny amount compared to more distant stars, and sensitive instruments can detect the change. Now for the second part of "parsec": arcsecond. In this instance, we're not referring to a measure of time. It's a part of a measurement of angle. Imagine the horizon around you broken up into 360 slices, or degrees. Each slice is about twice the width of the full moon. An arcminute is 1/60th of a degree, and an arcsecond is 1/60th of an arcminute. So astronomers measure the size of objects, or the parallax movement of stars in degrees, arcminutes and arcseconds. So, to put those terms together, a parsec is the parallax of one arcsecond. Just a warning, you're going to need to dust off your trigonometry for this. If you create a triangle, where one leg is the distance between the Earth and the Sun (one astronomical unit), and the opposite angle, measured by how far the star moves in the sky, is one arcsecond, the star will be 1 parsec away, the other leg of the triangle. For example, the closest star in the sky, Proxima Centauri, has a parallax measurement of 0.77233 arcseconds – that's how far it shifts in the sky from when the Earth shifts its position by 1 astronomical unit. If you put this into the calculation, you determine that Proxima Centauri is 1.295 parsecs away, or 4.225 light years. All articles from Astronomy Today (www.astronomytoday.com)