Earth is constantly bombarded by high energy particles from space, cosmic rays. Most are protons, the positively-charged particles that normally live in the nuclei of atoms. Many come from the sun, but the highest energy cosmic rays are an enigma. Some have scarcely believable energies, way beyond what could be produced by any conceivable particle accelerator here on Earth. We do not know where they came from, how they could get here, and we have no idea what process could give them such stupendous energies. Now a new observatory has allowed a collaboration of over 300 scientists to answer the first of these questions.
With an array of 1600 detectors that stretch over 3000 km2 of Argentina, the Pierre Auger Observatory detects the showers of particles generated when high energy cosmic rays whack into air molecules. The researchers have discovered that the origin of the highest-energy cosmic rays are quasars. All galaxies have black holes at their centre. In some galaxies gas and stars are falling into the central black hole and prodigious amounts of energy are radiated out as a result, turning the nucleus of the galaxy into a brilliant quasar. How quasars could generate such high energy cosmic rays is a total mystery.
We think the moon formed when a Mars-sized planet (Theia) smashed into the proto-Earth four and a half billion years ago. This remarkable idea is supported by the geology of moon rocks, brought back by the Apollo astronauts between 1968-1974, and by much more recent computer simulations.
But one puzzle remains: the relative proportions of oxygen isotopes are identical for Earth rocks and moon rocks. (Isotopes are different forms of an element that vary only in the mass of their atoms.) Originally this was taken to mean that the moon and Earth came from the same source — neatly supporting the impact hypothesis — until the computer simulations showed that most of the moon must be derived from Theia, not Earth. But Theia must have had a different proportion of oxygen isotopes to the proto-Earth.
This paradox is resolved by recent calculations. These show that the collision between Theia and proto-Earth was so violent that both bodies were extensively melted or vaporized and thoroughly mixed before Earth and moon condensed after just 1000 years. This eliminated variations in the proportions of oxygen isotopes.
This is exciting research as it implies that moon rocks may resemble the rocks of early Earth. Ironically we can’t find rocks from Earth’s earliest times here because plate tectonics and weathering has destroyed such ancient material. It seems that to learn about early Earth we should look to the moon.
Comet Holmes has dramatically increased in size over the last few weeks – it is now even bigger than the Moon in the night sky! However, it has also faded significantly – it is now a challenge to see from light-polluted London, but it is still easily visible from anywhere remotely dark. The photograph to the right was taken by planetarium presenter Tony Sizer on the night of 15 November 2007.
A spectacular sequence of images, taken by John Pane, shows exactly how Comet Holmes has changed over the last month.
What we believe happened is that the surface of the comet somehow ruptured, exposing fresh ice to the heat of the Sun. This ice then vapourises into a halo around the comet, similar to clouds in the sky on Earth. As we all know, clouds in the sky reflect sunlight, which is exactly what the cloud around this comet is doing. As the cloud increases in size, it appears brighter.
In Tony’s picture, you can actually see stars through the cloud of water surrounding the nucleus of the comet.
More details about the comet can be found in the Wikipedia entry.
If you you want to observe Comet Holmes yourself, look for a fuzzy object very close to the bright star Mirphak in Perseus. This map (click on it for a larger version) will help guide your eye towards Mirphak, high in the east in the evening – just look down from the easy-to-spot “W” of stars that form the constellation Cassiopeia.
Often in the science of astronomy, an image will appear that makes me quietly go “wow” in awe. The Japanese Aerospace eXploration Agency, JAXA, has produced one of those images from its Kaguya (Selene) lunar orbiter.
Kaguya entered lunar orbit a month ago. Its mission is to develop the technology and to gather scientific data for future lunar exploration.
Full details of the spectacular movie of the Earth setting behind the Moon can be seen in this JAXA press release.
“Earth-sets” do not occur naturally on the Moon – the Moon always presents the same face towards the Earth, and so if you were to stand on the side of the Moon that faced the Earth, the Earth would always appear in the same area of the sky. However, Earth-rise and Earth-sets can occur on spacecraft orbiting the Moon.
The first images were taken of the lunar surface by Kaguya in early November 2007, and they can be seen in this JAXA press-release… and they are equally stunning.
Even though the summer has long gone, we can still see the summer triangle!
The Sun is now setting so early that we are still able to see the constellations of Cygnus, Lyra & Aquila in the evening sky (see chart; click for a larger version) – and it is the brightest star in each of these constellations that make up the summer triangle. Although these constellations were directly overhead in the summer, they are now low down in the west, ducking beneath the western horizon at around 9pm. But they are still ideal for late afternoon or early evening observing.
The star that forms the head of Cygnus the swan is called Albireo, a blue and golden double star that is a beautiful sight through a small telescope. Notice how Cygnus is flying directly downwards, head-first into the western horizon! Just to the right of Albireo is the Ring Nebula in Lyra, the left over remains of a star that puffed out its outer layers to form the nebula some 1,500 years ago. And to the left of Albireo is the Dumbbell Nebula, another treat for small telescope owners.
Almost directly overhead, high in the east, is the easily recognisable W-shape of the constellation Cassiopeia (see chart, right; click for a larger version). Below Cassiopeia is Perseus, which contains the star Mirphak.
If you want to see a comet, look toward Mirphak. While all the stars are pin sharp, you will be able to see a fuzzy object just next to Mirphak – that is the incredible Comet Holmes! For more details about Holmes, see my previous posts. This is a rare opportunity to see a comet with the unaided eye, so do not miss out!
To the right of the W-shape, is the Andromeda Galaxy. It is the most distant object that can be seen with the unaided eye, a staggering 2.25 million light-years away! The hundred thousand million stars that make up the Andromeda galaxy are what allow us to see it over such a vast distance.
Looking to the east, you can see Orion rising from 9pm, meaning that winter is here!
In the chart to the left (click it for a larger map), Mirphak (with Comet Holmes passing by) is visible at the top-right, in the constellation of Perseus. In the constellation of Auriga to the bottom left is the bright star Capella. Below Perseus is Taurus, with the beautiful desert-island cluster of stars called the Pleiades. Also known as the seven sisters (because seven stars are just about visible to the unaided eye), the Pleiades are about the same size in the sky as the full Moon. In fact, the Moon is not too far away from the Pleiades on 23rd and 24th of November.
The planet Mars, the only planet that is sociable this month, can be seen to the left of Taurus, in the group of stars that form the constellation of Gemini.
And finally, we have the beautiful Orion. The red supergiant star to the top left is Betelgeuse, and to the bottom right is the white hot star Rigel. In the middle of both is Orion’s belt of three bright stars. Hanging down from the belt is a dagger of three fainter stars, at the centre of which is Orion’s nebulae – a cloud of gas and dust where new stars are forming.
The Orion nebula is easily visible to the unaided eye, and is a splendid sight to see even in a small telescope throughout the winter.
Comet Holmes continues to be clearly visible to the unaided eye. See my previous post for details of where to look (it hasn’t moved much in the last week or so).
For the latest photographs, some of which show a developing tail, see the SpaceWeather.com gallery (which is where we found the image to the left, by Vicent Peris and José Luis Lamadrid).
