Posts Tagged ‘white dwarf

The hottest white dwarf in the Galaxy

leave a comment »

The hottest white dwarf in the Galaxy

Diagram of the Milky Way showing our Sun, the white dwarf, and the gas cloud relative to our neighbor galaxy, the Large Magellanic Cloud (adajcent to it the Small Magellanic Cloud). The white dwarf RX J0439.8-6809 and the gas cloud are between us and the Large Magellanic Cloud. Credit: Philipp Richter/University of Potsdam

Astronomers at the Universities of Tübingen and Potsdam have identified the hottest white dwarf ever discovered in our Galaxy. With a temperature of 250,000 degrees Celsius, this dying star at the outskirts of the Milky Way has already even entered its cooling phase. The researchers also were the first to observe an intergalactic gas cloud moving towards the Milky Way — indicating that galaxies collect fresh material from deep space, which they can use to make new stars. These findings are published in the latest Astronomy & Astrophysics [Analysis of HST/COS spectra of the bare C–O stellar core H1504+65 and a high-velocity twin in the Galactic halo].

Written by physicsgg

November 27, 2015 at 2:05 pm

New Particle Born Inside Helium White Dwarf Stars

leave a comment »

The prediction could help explain some unexpected properties of helium white dwarfs

White dwarf stars are glowing embers, the remains of small stars that have run out of fuel to burn Most white dwarfs are hot lumps of charcoal, gradually radiating their heat into space. But a few are made of helium and it is these that we look at today.

Because they generate no heat of their own, white dwarfs will eventually become so cold that they should stop emitting significant amounts of heat and light. But this cooling process takes ages, far longer even than the age of the universe, so no so-called black dwarfs are yet thought to exist.

The precise rate at which white dwarfs cool depends on their internal structure. That’s fairly well understood for plain vanilla white dwarfs. But the helium flavour holds some surprises.

The conventional view is that helium under high pressure forms a plasma of nuclei in a sea of electrons. When the pressure increases, the nuclei become ordered forming a crystal. The properties of this helium crystal determine how quickly the star cools.

Recently, however, astrophysicists have pointed that the helium can also form a Bose-Einstein condensate. The question that Paulo Bedaque at the University of Maryland in College Park and a few pals investigate is how the presence of such a condensate might affect the properties of the star.

It turns out that helium condensates have an extraordinary rich behaviour in which various kinds of quasiparticles can form. These quasiparticles are essentially quantised excitations in the condensate and have been well studied for ordinary condensates.

Because these quasiparticles transport energy through and out of a condensate, they reduce its specific heat.

What Bedaque and co have found is an entirely new quasiparticle that emerges in helium white dwarfs because of the extra constraints on the behaviour of the condensate at the centre of such a dense object.

This quasiparticle, they say, reduces the specific heat of the white dwarf core by two orders of magnitude compared to a crystalline core.

The consequences aren’t difficult to fathom. The lower specific heat means that helium white dwarfs ought to cool down significantly faster than previously thought. In fact, Bedaque and co suggest this more rapid cooling ought to be detectable.

As it turns out, there is a known anomaly in the temperatures of helium white dwarfs. A couple of years ago, astronomers found a group of helium white dwarfs in a globular cluster a few thousand light years from here.

Plot the temperature and magnitude of white dwarfs and astronomers usually find that the sequence of stars becomes cooler and less bright until they are no longer visible to whatever telescope they are using.

But with these helium white dwarfs, astronomers saw something else: the sequence ended well above the magnitude limit of the observations (which were made with Hubble). For some reason, the dimmest, coolest, oldest stars aren’t where they are supposed to be

Nobody knows why but the new quasiparticle and the cooling it causes, raises a possibility. Perhaps helium white dwarfs go through some kind of internal transition as they get older that causes them to suddenly cool much faster than expected. That’s why the oldest coolest stars don’t follow the usual pattern of cooling.

For the moment, that’s just a guess. Bedaque and co say that a great deal more modelling is necessary to fully understand how all this might work out. More data on real helium white dwarfs would help too.

Looks like a few interesting months ahead for these guys.

Ref: Nuclear Condensate And Helium White Dwarfs

Written by physicsgg

November 11, 2011 at 3:23 pm

Ancient White Dwarf Stars

leave a comment »

Pushing the limits of its powerful vision, NASA’s Hubble Space Telescope uncovered the oldest burned-out stars in our Milky Way Galaxy in this image from 2002. These extremely old, dim “clockwork stars” provide a completely independent reading on the age of the universe without relying on measurements of the expansion of the universe.

The ancient white dwarf stars, as seen by Hubble, turn out to be 12 to 13 billion years old. Because earlier Hubble observations show that the first stars formed less than 1 billion years after the universe’s birth in the big bang, finding the oldest stars puts astronomers well within arm’s reach of calculating the absolute age of the universe.

Though previous Hubble research sets the age of the universe at 13 to 14 billion years based on the rate of expansion of space, the universe’s birthday is such a fundamental and profound value that astronomers have long sought other age-dating techniques to cross-check their conclusions.

The new age-dating observations were done using Hubble to hunt for elusive ancient stars hidden inside a globular star cluster located 5,600 light-years away in the constellation Scorpius.

Written by physicsgg

November 2, 2011 at 9:15 pm