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Question Of The Day: Black dwarfs – how do these invisible stars form?

Robert Jones — Sat, 06 Aug 2011 07:00:31 +0000

When a star becomes a white dwarf it no longer has an internal heat source, instead staying luminous only because of its retained temperature. A black dwarf is a white dwarf that has given off all its heat and cooled down to the temperature of the surrounding universe, known as the cosmic microwave background, which is about 2.7 Kelvin.

While a white dwarf gives off heat via thermal radiation it does so incredibly slowly, because the densely packed electrons that prevent it from collapsing are excellent conductors of heat. A hypothetical white dwarf born at the start of the universe 13.7 billion years ago wouldn’t have cooled down to the temperature required to form a black dwarf even today. In fact, the estimated time for a white dwarf to cool to a black dwarf would be roughly 73,000 times the age of the universe. For this reason, unlike their white, red and brown dwarf cousins, black dwarfs have not yet been observed. Indeed, as a black dwarf would emit little to no radiation, finding one would be nigh on impossible, as it would appear almost invisible to us, other than the effects of its gravity.

Answered by HIW.

Cool iceberg action close up

Helen — Thu, 11 Feb 2010 14:18:42 +0000

Man captures rare close-up footage of an iceberg collapsing

Capturing an iceberg collapsing or calving is a rare sight, and this couple filming off Labrador, Canada, picked up their camcorder just in time to catch the dramatic event. The pair and their dog are clearly excited about what they’ve just witnessed, but are soon chased away by a rush of smashed up ice that starts floating towards them. For an in-depth look at how icebergs are formed check out issue five of How It Works, which goes on sale on 25 February. By the way, when you watch this video just try not to imagine Kermit the Frog is behind the camera. You’ll see what we mean.

The Mayan Underworld explored

Robert Jones — Fri, 15 Jan 2010 12:44:46 +0000

For life to prosper water is a fundamental necessity, but if so, how did the the ancient Maya build such a massive empire without a river to draw water from? This, the first of five videos from the BBC documentary Secrets of the Mayan Underworld, demonstrates how through a series of underground, water-filled caves that run under the majority of the Yucatan, the Mayan Underworld – contrary to their belief that it was a realm of spirits – was actually the life-source of the entire area.

Despite being one of the most active and populated centres of civilisation in the Classic Period (250 AD to 900 AD), with advanced mathematical and astronomical systems, the ancient Maya went into a sharp decline in the 8th and 9th centuries. Many theories for this collapse have been postulated by scholars, both ecological and non-ecological, however no firm agreement has been reached. Many theories centre around over-population, climate change and epidemic disease, as well as foreign invasion, mass underclass revolt and the collapse of key trading routes. Recently academics theorised that a 200 year drought was a major contributor to this mighty civilisation’s break down.