But when you see animations of galaxies , especially as they come together and collide, you see the stars buzzing around like angry bees. Stars, of course, do move. But astronomers have been studying their position for thousands of years. Tracking the position and movements of the stars is known as astrometry. We trace the history of astrometry back to BC, when the ancient Greek astronomer Hipparchus first created a catalog of the brightest stars in the sky and their position.
His student Ptolemy followed up with his own observations of the night sky, creating his important document: the Almagest.
In the Almagest, Ptolemy laid out his theory for an Earth-centric Universe, with the Moon, Sun, planets and stars in concentric crystal spheres that rotated around the planet. He was wrong about the Universe, of course, but his charts and tables were incredibly accurate, measuring the brightness and location of more than 1, stars. A thousand years later, the Arabic astronomer Abd al-Rahman al-Sufi completed an even more detailed measurement of the sky using an astrolabe.
One of the most famous astronomers in history was the Danish Tycho Brahe. He was renowned for his ability to measure the position of stars, and built incredibly precise instruments for the time to do the job.
He measured the positions of stars to within 15 to 35 arcseconds of accuracy. Just for comparison, a human hair, held 10 meters away is an arcsecond wide.
He lost his in a duel, but had a brass replacement made. In , Friedrich Bessel was the first astronomer to measure the distance to a nearby star 61 Cygni.
He used the technique of parallax, by measuring the angle to the star when the Earth was on one side of the Sun, and then measuring it again 6 months later when the Earth was on the other side. Over the course of this period, this relatively closer star moves slightly back and forth against the more distant background of the galaxy.
And over the next two centuries, other astronomers further refined this technique, getting better and better at figuring out the distance and motions of stars.
But to really track the positions and motions of stars, we needed to go to space. The angle between two points in the sky is defined as the angle between two imaginary lines running from you out to those points. For the two stars shown, the angle is about 16 degrees. The bigger the angle, the farther apart the two points appear to be in the sky. The actual distance between two stars is much harder to determine, as we'll later see. To measure the angles between stars and other points in the sky, astronomers use protractors and similar instruments, often attached to a telescope for accurate pointing.
To get an approximate measurement, however, you can use instruments that are always with you: your hands. These angles don't depend much on your size, because people with bigger hands also tend to have longer arms.
Next time you see the Big Dipper, hold out your fist and check that the Dipper's bowl is about one fist wide. To estimate larger angles you can use both hands to count multiple fists.
Question: How many fists, stacked one on top of another, would it take to reach from the horizon to zenith? Now look back at the east- and west-facing star trail photos at the top of this page.
The stars in these photos are following circular arcs that begin in the east, pass high across the southern sky, and end in the west. You, the observer, are at the approximate center of these circular arcs, so you can directly measure the angle through which these stars move, by holding up your hands to the real sky, not the photo!
If you make this measurement carefully, you'll find that in 10 minutes, each of these stars moves through an angle of 2. Over a full hour day, the angle of rotation would be. Of course, you normally can't see the stars during daylight, but they're still there and still following their circular paths, as you can confirm with a telescope or by getting above earth's atmosphere. Question: How many minutes would it take for a star to move just one degree? Calculate the answer carefully—don't just guess.
The rate of angular motion is the same in other parts of the sky, although you can't just measure the angles with your hands because you're not at the center of the circles. In the northern sky, however, you can measure the angles directly by laying a protractor down on a photograph.
Here's a longer time exposure of star trails near the North Star: In the northern sky, all stars move at the same rate around the common center of their circles. Question: How would you use the data from the preceding photo to calculate the time required for a one-degree rotation? This computer-simulated multiple-exposure image made with Sky Motion Applet shows Orion in the southern sky at the same time on seven successive nights. Each night, after completing a full circle, the stars have shifted rightward by about one degree.
To be precise, though, I need to tell you that all of the angles quoted above are only approximate. In fact, it takes just 23 hours and 56 minutes, or four minutes less than a full day. If you really want to be precise about these things, you also need to take into account leap years—but let's not bother.
So, as the seasons pass, we see different groups of stars in a given direction, at any given time of night. In January you can watch Orion rising in the east just after sunset, but by March, Orion will be high in the south, heading westward, by the time the sky is dark.
Meanwhile the bright star Arcturus will be rising in the east , a sign that spring is coming. If you learn to identify the prominent stars and constellations, they will give you a strong sense of the passage of the seasons.
Night owls and early risers can also enjoy a preview of the stars that evening observers will see in the coming months. To simplify their understanding of the motions of the sky, ancient people invented a mechanical model to explain these motions. We still use this model today because it's so convenient—even though it's wrong. If you can visualize the model, you won't have to memorize a whole bunch of separate facts about how the stars move.
The stars appear to be attached to a giant celestial sphere, spinning about the celestial poles, and around us, once every 23 hours and 56 minutes. The model is simply that the stars are all attached to the inside of a giant rigid celestial sphere that surrounds the earth and spins around us once every 23 hours, 56 minutes. The spinning carries each star around in its observed circular path, while a special point in the northern sky, at the center of the circles, remains fixed.
The sphere's rigidity accounts for how the shapes of the constellations never change, and its enormous size accounts for how the constellations never grow or shrink, as they would if a particular point on earth were significantly closer to one side of the sphere than the other. To better describe locations in the sky, we give names to the various parts of the celestial sphere. The fixed point in the northern sky is called the north celestial pole , and is located only about a degree away from the famous North Star which makes tiny circles around it.
Ninety degrees from the pole is the celestial equator , a great circle that runs from directly east to directly west, passing high above our southern horizon. Mintaka , the rightmost star in Orion's Belt, happens to lie almost exactly on the celestial equator, so you can think of the celestial equator as tracing the path of this star.
Another important great circle is the meridian , which runs from directly north to directly south, passing straight overhead. As the sphere turns, the meridian remains fixed in the sky. The point straight overhead is called zenith. The ejected pair evolved until one component became a red giant and the two spiraled closer bottom left.
To B or not to B So far, almost all known hypervelocity stars are B-type suns on the main sequence, the period in their lives when they produce energy by fusing hydrogen into helium in their core.
Unless it was ejected there, you would never expect to see a B star traveling at those speeds in the outer halo. The best explanation for their existence involves a binary star that ventures too close to a massive black hole, says Hagai Perets, an astrophysicist at the Technion — Israel Institute of Technology in Haifa. The black hole captures one star into a highly eccentric orbit and ejects the other as a hypervelocity star.
The higher the velocity, the greater the shift. Still, they would be easier to detect than the even fainter white dwarf remnants of any dead B-type star. Los Alamos theorist Jack Hills predicted hypervelocity stars in , but astronomers didn't find one until But even this tells only half the story. This so-called proper motion is even harder to measure precisely than radial velocity. For a hypervelocity star, this means measuring its movement in relation to background galaxies or quasars, a process that takes years.
Despite their breakneck speeds, hypervelocity stars have proper motions of less than 1 milliarcsecond per year. One milliarcsecond equals 0. Ground-based surveys are accurate to only about 5 milliarcseconds per year, so proper-motion studies for hypervelocity stars must be done from space. This astrometric observatory — designed to measure precise positions and radial velocities of some 1 billion stars — is yielding proper motions accurate to within 0. In the next year or two, Gaia should provide superb proper motions for known hypervelocity stars and new candidates.
Although the source remains elusive, Brown argues that the star is a remnant of a binary system ejected from the Milky Way. His scenario begins with a trio of stellar companions: a tightly bound binary pair in orbit with a more-distant sun. As the pair hurtled away from the galactic center, the more massive star eventually evolved into a red giant. Otherwise, this particular star would have evolved off the main sequence long ago. Heber preferentially targets relatively low-mass stars that have evolved into bloated red giants.
Such stars burn helium in their cores rather than hydrogen. In the second scenario, two stars revolve around each other in a tightly bound orbit. When the higher-mass one reaches the end of its life and its core collapses, it triggers a supernova that can liberate its lower-mass companion. This mechanism will work wherever young stars hang out, including inside youthful star clusters.
The neutron star RX J— is a prime example. In , astronomers clocked it moving at 1. The explosion that created Puppis A — a supernova remnant some 7, light-years from Earth in the southern constellation Puppis — launched the stellar remnant onto this trajectory. Astronomers think this supernova was a lopsided explosion, and the neutron star headed one way while much of the supernova debris went in the opposite direction. Thus far, the fastest of these hypervelocity stars have been clocked at about 2 million miles per hour.
But US is moving at more than 26 million miles per hour. Geier and some colleagues first identified US in In the new work, he and his co-authors were able to measure the speed of the star by using both current and archival data, and watching its motion change over a total of about 70 years.
The monster black hole at the center of the Milky Way has the gravitational muscle to fling a star on a one-way-track out of the neighborhood, and many other hypervelocity stars are thought to originate from there. But US didn't start its journey near the galactic center, the new research shows. Based on additional clues, the scientists say it was probably orbiting another star when its path changed.
US and its partner star were likely orbiting each other very quickly, with a very small distance between them. The neighbor star exploded into a supernova and was completely destroyed. US was suddenly without a gravitational tether to keep it in the same place, and all that rotational speed and energy then abruptly started moving in a straight line.
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