The origin of type Ia supernovae, the standard candles used to reveal the presence of dark energy in the universe, is one of astronomy’s most beguiling mysteries. Astronomers know they occur when a white dwarf explodes in a binary system with another star, but the properties of that second star — and how it triggers the explosion — have remained elusive for decades.
Now, a team of astronomers from the intermediate Palomar Transient Factory (iPTF), including those associated with UC Santa Barbara, have witnessed a supernova smashing into a nearby star, shocking it, and creating an ultraviolet glow that reveals the size of the companion. The discovery involved the rapid response and coordination of iPTF, NASA’s Swift satellite and the new capabilities of the Las Cumbres Observatory Global Telescope Network (LCOGT).
Using NASA’s Hubble Space Telescope, astronomers have captured for the first time snapshots of fledging white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs.
White dwarfs are the burned-out relics of stars that rapidly lose mass, cool down and shut off their nuclear furnaces. As these glowing carcasses age and shed weight, their orbits begin to expand outward from the star cluster’s packed downtown. This migration is caused by a gravitational tussle among stars inside the cluster. Globular star clusters sort out stars according to their mass, governed by a gravitational billiard ball game where lower mass stars rob momentum from more massive stars. The result is that heavier stars slow down and sink to the cluster’s core, while lighter stars pick up speed and move across the cluster to the edge. This process is known as “mass segregation.” Until these Hubble observations, astronomers had never definitively seen the dynamical conveyor belt in action.
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“Up above the world so high, like a diamond in the sky…” A team of astronomers, using multiple telescopes, has identified the coolest, faintest white dwarf star known. White dwarfs are the extremely dense end states of stars like our sun: after their nuclear fuel is exhausted, they collapse from the size of a star (about 1,000,000 miles across) to the size of the Earth (7,000 miles across). This white dwarf, located in the constellation Aquarius, is so cool that its carbon has crystallized—in other words, it’s like a diamond, with a mass similar to that of our sun.
The path to this discovery began when Dr. Jason Boyles, then a graduate student at West Virginia University, identified what astronomers refer to as a millisecond pulsar in this location. Pulsars are spinning neutron stars—the collapsed end state of a star many times more massive than our sun, but only about 20 miles across. Known as PSR J2222-0137, which simply identifies its position in the sky, this pulsar is spinning over 30 times a second. Its orientation is such that as it spins, a beam from its magnetic pole sweeps repeatedly past the earth, giving rise to regular blips of radio waves. (The pulsar is detected only in radio waves, not in visible light.) The observations also revealed that this pulsar is gravitationally bound to a companion star: the two orbit around each other every 2.45 days. It is this companion object that appears to be either another neutron star or, more likely, a remarkably cool white dwarf.
Astronomers from The University of Texas at Austin and colleagues have used the 2.1-meter Otto Struve Telescope at the university’s McDonald Observatory to discover pulsations from the crystalized remnant of a burnt-out star. The finding will allow astronomers to see below the star’s atmosphere and into its interior, much like earthquakes allow geologists to study compositions below Earth’s surface. The findings appear in the current issue of The Astrophysical Journal Letters.
The Texas astronomers made their discovery in collaboration with astronomers from Brazil’s Universidade Federal do Rio Grande do Sul, the University of Oklahoma, and the Smithsonian Astrophysical Observatory.
The star, GD 518, is roughly 170 light years from Earth in the constellation Draco, but far too faint to be seen without a telescope. It is a white dwarf, a star at the end of its life cycle that is essentially just a burnt-out core, the ashy byproduct of previous epochs of nuclear fusion.
The star is unique in that much of it is likely suspended in a state more akin to a solid than a liquid or gas. The interiors of dying stars can become crystalized similar to the way in which frigid water freezes into ice, like the slow formation of glaciers in cooling ocean water.
The NASA/ESA Hubble Space Telescope has found signs of Earth-like planets in an unlikely place: the atmospheres of a pair of burnt-out stars in a nearby star cluster. The white dwarf stars are being polluted by debris from asteroid-like objects falling onto them. This discovery suggests that rocky planet assembly is common in clusters, say researchers.
The stars, known as white dwarfs — small, dim remnants of stars once like the Sun — reside 150 light-years away in the Hyades star cluster, in the constellation of Taurus (The Bull). The cluster is relatively young, at only 625 million years old.
Astronomers believe that all stars formed in clusters. However, searches for planets in these clusters have not been fruitful — of the roughly 800 exoplanets known, only four are known to orbit stars in clusters. This scarcity may be due to the nature of the cluster stars, which are young and active, producing stellar flares and other outbursts that make it difficult to study them in detail.
Among the hundred billion stars which can be observed in the Milky Way, there is a group of stars, the so-named ultra-cool dwarfs, defined as stars with a temperature below 2500 K, which includes ultra-cool dwarfs and brown dwarfs. It is a really interesting group: they are the most ancient objects in our Galaxy and, therefore, they can provide information about its primitive chemical composition. This is one of the objectives of Gaia mission which will be launched at the end of 2013 by the European Space Agency.
When observing them, they seem quite similar, but there are clear differences between brown dwarfs and ultra-cool dwarfs: brown dwarfs do not reach the temperature they need to produce the nuclear reactions which characterize ultra-cool dwarfs. It could be said that brown dwarfs are failed stars because they lack mass.
A study published on the journal Astronomy & Astrophysics, led by the National University of Distance Education (UNED) and in which researchers from the Institute of Cosmos Sciences of the UB (ICCUB) participated, has developed a method that will allow Gaia to detect tens of ultra-cool dwarfs in the Milky Way. The method to estimate physical parameters of these objects, such as temperature or gravity, has also been validated. Researchers have used data mining techniques to make estimations taking into account the parameters that Gaia can measure and its design characteristics.
Astronomers find planets in strange places and wonder if they might support life. One such place would be in orbit around a white or brown dwarf. While neither is a star like the sun, both glow and so could be orbited by planets with the right ingredients for life.
No terrestrial, or Earth-like planets have yet been confirmed orbiting white or brown dwarfs, but there is no reason to assume they don’t exist. However, new research by Rory Barnes of the University of Washington and René Heller of Germany’s Leibniz Institute for Astrophysics Potsdam hints that planets orbiting white or brown dwarfs will prove poor candidates for life.
White dwarfs are the hot cores of dead stars and brown dwarfs are failed stars, objects not massive enough to start nuclear burning as the sun does. In theory, both can be bright enough to theoretically support a habitable zone — that swath of space just right for an orbiting planet’s surface water to be in liquid form, thus giving life a chance.
White and brown dwarfs share a common characteristic that sets them apart from normal stars like the sun: They slowly cool and become less luminous over time. And as they cool, their habitable zones gradually shrink inward. Thus, a planet that is found in the center of the habitable zone today must previously have spent time near the zone’s deadly inner edge.