{"id":33549,"date":"2022-07-20T00:00:00","date_gmt":"2022-07-20T00:00:00","guid":{"rendered":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/?post_type=purple_issue&p=33549"},"modified":"2022-07-21T14:11:12","modified_gmt":"2022-07-21T14:11:12","slug":"cutting-edge-keeping-cool-when-youre-a-white-dwarf","status":"publish","type":"post","link":"https:\/\/c01.purpledshub.com\/bbcskyatnight\/2022\/07\/20\/cutting-edge-keeping-cool-when-youre-a-white-dwarf\/","title":{"rendered":"Cutting edge: Keeping cool when you\u2019re a white dwarf"},"content":{"rendered":"\n
Our experts examine the hottest new research<\/p>\n\n
CUTTING EDGE<\/span><\/h2>\n\n\n\n
Keeping cool when you\u2019re a white dwarf<\/h2>\n\n
New research reveals that the slow cooling of white dwarfs is far more complex than it first appears<\/h4>\n\nHelium\u2019s idiosyncrasy under high density held the answer to a decades-old question <\/figcaption><\/figure>\n\n
White dwarfs have always struck me as, well, a little dull. Compared to the neutron stars and black holes produced by more massive stars at the end of their lives, the fact that our Sun will end its days as a \u2018mere\u2019 white dwarf seemed slightly disappointing. These stellar embers come across as simple, doing little but cooling and fading during their long afterlives.<\/span><\/p>\n\n
It turns out that\u2019s nonsense. Dense balls of matter held up by weird complex effects, created in the maelstrom at the end of a star\u2019s life, it shouldn\u2019t be a surprise that white dwarfs are as complex and fascinating as any other star. This month\u2019s paper, from a Canadian\u2013American team, sheds light on some of the weirder members of the white dwarf family. <\/p>\n\n
The best way to understand stars is to make a Hertzsprung-Russell diagram, plotting the colour (or temperature) against brightness. In such a diagram, stars like the Sun form a distinct line, known as the \u2018main sequence\u2019, which stretches from bright, hot blue stars at one end to faint, cool red dwarfs at the other. Off to one side, however, are the white dwarfs which are on the blue-to-white end, but faint. Yet it turns out that the white dwarfs are not just scattered randomly across their bit of the diagram. <\/p>\n\n
Cooling-off period <\/strong><\/h4>\n\n
If you\u2019re a white dwarf, your main activity is to spend time cooling down. How you do that depends on your age and state. Early on, the white dwarf efficiently loses energy by radiating, but later on things become more complicated. The sample studied here, for example, seems to be cooling via a process which involves collisions between hydrogen and helium in their atmospheres. <\/p>\n\n
White dwarf atmospheres are deeply odd, forming a thin layer around the surface of the star. If the Earth had such an atmosphere, the largest skyscrapers would stick out of the top. In such circumstances, collisions between atoms and molecules are common. When molecules collide, they can radiate, releasing energy, and a set of white dwarfs are believed to use this method to become ultracool white dwarfs.<\/span><\/p>\n\n
\u201cThe great breakthrough is to realise that helium ions behave differently at the densities reached in the atmosphere of a white dwarf\u201d<\/p><\/blockquote>\n\n
The trouble was that using this effect to explain the position of these ultracool dwarfs in the HR diagram failed to match their observed temperatures. <\/p>\n\n
The great breakthrough of this work \u2013 solving a problem that has bothered astronomers for decades \u2013 is to realise that helium ions behave differently at the densities reached in the dense but narrow at mosphere of a white dwarf. It\u2019s a good example of why studying these places is interesting: they make us think about conditions that don\u2019t exist elsewhere and test our theories. <\/p>\n\n
Correcting the effect of helium ions on the atmosphere makes everything work. The white dwarfs in question are more massive than we thought, perhaps nearly as massive as the Sun, and they sit neatly on the HR diagram. The only problem is explaining where the hydrogen in the ones with the most massive atmospheres comes from. One possibility is that they\u2019ve accreted it from in-falling asteroids, comets and even planets \u2013 further proof that life for white dwarfs is far from boring. <\/p>\n\n
\n\n
<\/figure><\/div>\n\n
Prof Chris Lintott is an astrophysicist and co-presenter on The Sky at Night<\/em>.<\/p>\n\n
![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/77\/2022\/07\/d9f1c82f-13ce-4784-a218-38af9eb39df0.jpg\")
![\"\"](\"https:\/\/dj9jqhxgw9833.cloudfront.net\/uploads\/sites\/77\/2022\/03\/80T3Q09C3T0J5QL98J2RRJZLU761-1024x1024.jpg\")
<\/figure><\/div>\n\n