Posts tagged stars
Posts tagged stars
Distance: 23,000 light years away
In 1967, suspiciously regular pulses of radiation were detected coming from space – so regular that their discoverers thought they could be signals from an alien civilisation. That hypothesis was soon abandoned and the source was named a pulsar, or pulsing star. Since then, the metronomic emissions of gamma rays, X-rays or radio waves from pulsars has made them cosmic chronometers.
That’s why Fernando Camilo, an astronomer at Columbia University in New York, was astounded when the radio pulsar he had discovered and had been observing for a year – PSR J1841-0500 – suddenly stopped beaming its regular bursts. “At first I had a hard time believing what I was seeing,” he recalls. “For the past year, the pulsar had been so reliable, pulsing brightly once every 1.9 seconds. I thought there must be an error with the equipment.”
Intrigued, Camilo kept observing the star at 5-minute intervals once a month either at the CSIRO Parkes Observatory in New South Wales, Australia, where it had been discovered, or at the National Radio Astronomy Observatory in Green Bank, West Virginia. A year and a half later, his hard work paid off when the star came back to life, pulsing just as brightly as before.
Other pulsars have switched off for short periods, but Camilo’s has taken by far the longest break ever seen, raising new questions about just how reliable these cosmic clocks are. The finding also adds to the mystery surrounding pulsars, as exactly what makes them tick in the first place isn’t well understood either.
From a typical pulsar with a frequency of 1.40 rotations/second, to the second known fastest rotating pulsar with a frequency of 642 rotations/second.
Just as the Sun rises in the east and sets in the west, so do the stars appear to slowly march across the sky. Their leisurely pace is imperceptible to a casual observer, but you can test the effect for yourself: on the next clear night note the position of a bright star, and then check again a few hours later. The change is not caused by the motion of the stars themselves, but rather the rotation of the Earth.
Long-exposure photography is the ideal way to capture this motion. A camera is set up on a tripod, and the shutter opened to the sky. Normal snapshots gather light for a fraction of a second, but these special images need starlight to pour onto them for much longer, like a bucket collecting rainwater.
To obtain this image, ESO Photo Ambassador Gianluca Lombardi collected light for a total of 25 minutes. This may not seem like a long time, but the streaks of light in the night sky tell a different story. The Earth has rotated so that the pin-pricks of starlight have become star trails. In the top left, the trails form arcs around the southern celestial pole, which is outside the photograph. The ghostly traces of someone walking across the Paranal observing platform can also be seen.
Many familiar and outstanding pictures of astronomical objects are obtained using the same principle of accumulating light over a long period of time to build up an image. It is common for telescopes to gather light for several hours to make a single picture. This brings with it an additional challenge: the Earth rotating means that the telescope must also move to keep track of its target.
To examine the way the universe behaved in the past, astronomers look at extremely distant objects, such as supernovae in galaxies billions of light-years away. But how does that work? How can astronomers look out into space and see the universe back in time?
The answer lies in the speed of light. Light waves move very fast, about 186,000 miles per second (300,000 km/s). Light moves so fast that as you go about your daily life, it appears to travel instantaneously from one place to another. For example, it takes only a few billionths of a second for light to travel across your bedroom when you turn on a lamp.
In space, however, the distances are so immense that the time that light takes to travel is noticeable.
The Moon is Earth’s closest companion, at about 239,000 miles (390,000 km) away. Light takes around 1.3 seconds to travel that distance.
The Sun is 93 million miles (150 million km) away, far enough that the light it emits needs about 500 seconds to travel to Earth. We call the distance light takes to travel in a second a light-second, the distance it takes to travel in a minute a light-minute, and so on. So the Sun is about eight light-minutes away from Earth. The light shining on you right now first left the Sun eight minutes earlier.
Across our Milky Way galaxy, distances are measured in terms of how many years it takes light to travel. The nearest star is over four light-years away. So when we look at that nearest star, we see it not as it is today, but as it was four years ago. We are seeing the light that left that star four years previously and is just reaching us now.
The diameter of our galaxy is 100 thousand light-years. So when we look at even more distant stars, we see them as they were thousands to tens of thousands of years ago, depending on how far away they are and thus the distance their light has had to travel.
Galaxies are yet farther away in both space and time. Our nearest large neighbor galaxy, Andromeda, is about two and a half million light-years away. The Virgo Cluster of galaxies is the largest nearby collection of galaxies, at about 60 million light-years from the Milky Way. The light we see today from galaxies in the Virgo Cluster started on its path toward us at the same time as the age of the dinosaurs was ending on Earth. If you were in a Virgo Cluster galaxy today, and you had a telescope powerful enough to study Earth, you would be able to see the prehistoric reptiles.
Very distant galaxies are billions of light-years away. At that distance, their light tells what the universe was like billions of years ago. Since the age of the universe is about 14 billion years, these distant observations allow astronomers to measure changes over the lifetime of the universe. So when astronomers look out into space, they are essentially also looking back into time.
This fact was vital to the teams studying the expansion of space, because their goal was to compare the speed of the universe’s expansion in the past with the speed of the universe today. By studying extremely distant supernovae in faraway galaxies, they were able to judge the speed of the universe’s expansion in the early universe.
13:41 20 May 2011 by David Shiga
Our home galaxy is in the midst of a mid-life crisis, with the bulk of its star-formation behind it, a new study suggests. An impending merger with another galaxy will provide only a brief flurry of activity in an otherwise dull future.
Most galaxies fall into one of two camps: blue galaxies that form stars vigorously and are full of young, blue stars, and red galaxies that produce stars sluggishly or not at all and are dominated by older, red stars.
Galaxies of intermediate colour, called “green valley” galaxies, are relatively rare. They are thought to be in the process of changing from blue to red, with star-formation waning.
A new study by Simon Mutch of the Swinburne University of Technology in Hawthorn, Victoria, Australia, and colleagues suggests our own galaxy is experiencing such a decline. It appears to have entered the green valley, with a future as a red, dead galaxy looming on the horizon.
The Milky Way’s overall colour is difficult to determine from our position inside it, as dust clouds create “blind spots” that block visible light from many of its regions. However, infrared observations, which can penetrate dust, have revealed that its star formation rate is unexceptional, too low to put it clearly in the blue group and too high to be unequivocally red.
To figure out what stage of life our galaxy is in, Mutch’s team simulated the formation and evolution of 25 million galaxies and selected those similar to the Milky Way in terms of their star formation rate, shape and the total mass of their stars.
The researchers found that these simulated Milky Ways were mostly green valley galaxies, suggesting that the real Milky Way is in this transitional state too, the team reports in a paper to appear in the Astrophysical Journal.
Past outbursts from the massive black hole at the Milky Way’s centre may be to blame for this mid-life crisis. Radiation “burps” produced after matter fell into the black hole could have heated and expelled gas from the galaxy, the team says.
So when will the Milky Way put its star-forming years behind it? If it behaves like the galaxies in the simulation, it will spend just 1.5 billion years in the green valley before going red.
But Rosemary Wyse of Johns Hopkins University in Baltimore, Maryland, says this mid-life period could be extended in the real Milky Way. Some gas clouds – which could go on to form stars – appear to be falling onto the galaxy. Still, she says: “Star formation probably will decrease with time.”
And there may be a brief burst of star formation in 5 billion years, when the Milky Way is expected to merge with Andromeda, a nearby spiral galaxy whose properties also indicate it is a green valley galaxy, the team says.
The merger may send residual gas towards the centre of the merged object, triggering a short stellar baby boom before the galaxy settles into old age, says team member Darren Croton, also at Swinburne. The spiral discs of both galaxies will be destroyed in the collision, he says: “The violence of the merger will cement our place as a red and dead elliptical galaxy.”
In case this sounds depressing, he notes that red elliptical galaxies are not completely devoid of activity. “Many of them continue to lead very interesting lives in their retirement,” he says, pointing to a nearby elliptical called M87 that has spectacular jets streaming from its central black hole.
(From 2008) This Chandra X-ray Observatory image shows Westerlund 2, a young star cluster with an estimated age of about one or two million years. Until recently little was known about this cluster because it is heavily obscured by dust and gas. However, using infrared and X-ray observations to overcome this obscuration, Westerlund 2 has become regarded as one of the most interesting star clusters in the Milky Way galaxy. It contains some of the hottest, brightest and most massive stars known.
This Chandra image of Westerlund 2 shows low energy X-rays in red, intermediate energy X-rays in green and high energy X-rays in blue. The image shows a very high density of massive stars that are bright in X-rays, plus diffuse X-ray emission.
An incredibly massive double star system called WR20a is visible as the bright yellow point just below and to the right of the cluster’s center. This system contains stars with masses of 82 and 83 times that of the Sun. The dense streams of matter steadily ejected by these two massive stars, called stellar winds, collide with each other and produce copious amounts of X-ray emission. This collision is seen at different angles as the stars orbit around each other every 3.7 days. Several other bright X-ray sources may also show evidence for collisions between winds in massive binary systems.
Read entire caption/view more images: www.chandra.harvard.edu/photo/2008/wd2/
Image credit: NASA/CXC/Univ. de Liège/Y. Naze et al
By John Roach
No doubt, naked-eye views of the universe are spectacular, but there’s much more going on out there than appears in visible light. That’s why astronomers routinely observe with a variety of telescopes equipped to capture multiple wavelengths of light across the electromagnetic spectrum.
To showcase how these different wavelengths highlight particular features and processes in the lifecycle of stars, the European Space Agency trained their fleet of space telescopes on the Andromeda Galaxy, also known as M31, and compiled the observations into this video.
To get the star party started, the video shows where the Andromeda Galaxy is in an optical – visible light view – of the night sky and then zeros in on it with a microwave view from the Planck spacecraft. Microwaves are at the long-wavelength end of the spectrum and are sensitive to particles of extremely cold dust – just a few tens of degrees above absolute zero.
After a shift to a close-up optical shot of the galaxy, located about 2.5 million light years away, we are treated to an infrared view observed by the Herschel space telescope, which highlights slightly warmer dust. This dust traces locations in the spiral arms of Andromeda where new stars are being born today, the ESA notes in a media advisory.
The XMM-Newton telescope detects wavelengths shorter than visible light, collecting ultraviolet and X-rays. These show older stars, many nearing the end of their lives and others that have already exploded, sending shockwaves through space. ESA has used this telescope to monitor the core of Andromeda since 2002, revealing many variable stars, some which have exploded as novae.
The ultraviolet views display light from extremely massive, young stars that have a relatively short life span: They exhaust their nuclear fuel and explode as supernovae typically within a few tens of millions of years after they are born. Ultraviolet light is usually absorbed by dust and re-emitted as infrared. That means the areas where ultraviolet light is directly seen are relatively clear, dust free regions of Andromeda.