What is the Aurora Borealis?

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The aurora (plural aurorae) borealis has many other names: northern lights, northern polar lights, polar lights, and more. An aurora borealis is light seen in the sky, nearly always at night, in the northern hemisphere, commonly green but also red and (rarely) other colors; often in the shape of curtains, sheets, or a diffuse glow (when seen from the ground). Northern lights are most often seen at high latitudes – Alaska, Canada, northern Scandinavia, Greenland, Siberia, and Iceland – and during maxima in the solar cycle.

Aurora australis – southern lights – is the corresponding southern hemisphere phenomenon.

Seeing a bright auroral display may be on your list of ‘things to see before I die’! Yep, they are nature’s light show par excellence.

Aurora borealis occur in the Earth’s ionosphere, and result from collisions between energetic electrons (sometimes also protons, and even heavier charged particles) and atoms and molecules in the upper atmosphere. The ultimate origin of the energy which powers the aurora borealis is the Sun – via the solar wind – and the Earth’s magnetic field. Interactions between the solar wind (which carries its own tangled magnetic fields) and the Earth’s magnetic field may cause electrons (and other particles) to be trapped and accelerated; those particles which do not escape ‘downstream’ to the magnetic tail ‘touch down’ in the atmosphere, close to the north magnetic pole.

The different colors come from different atoms or ions; green and red from atomic oxygen, nitrogen ions and molecules make some pinkish-reds and blue-violet; purple is the appearance of combined colors from nitrogen ions and helium; neon produces the very rare orange. The ionosphere is home to most aurorae borealis, with 100-300 km being typical (this is where green is usually seen, with red at the top); however, some particularly energetic particles penetrate much deeper into the atmosphere, down to perhaps 80 km or lower (purple often comes from here).

Viewed from space, when the northern lights are intense they appear as a ring (an oval actually), the auroral zone, with the north magnetic pole near the center.

The University of Alaska Fairbanks’ Geophysical Institute has a good FAQ on the aurora borealis.

Magnetic fields plus solar wind … so you’d expect aurorae on Jupiter and Saturn, right? And auroral displays around the magnetic poles of these planets are now well documented. Aurorae have also been imaged on Venus, Mars, Uranus, Neptune, and even Io.

Some Universe Today stories on aurorae – borealis, australis, … and extra-terrestrial: What are the Northern Lights?, Aurora Reports from Around the World, Behind the Power and Beauty of the Northern Lights, Northern & Southern Aurorae Are Siblings, But Not Twins, Two Rockets Fly Through Auroral Arc, Chandra Looks at the Earth’s Aurora, First Aurora Seen on Mars, and Saturn’s “Dualing” Aurorae.

What Is The Crab Nebula?

The Crab Nebula, or M1 (the first object in Messier’s famous catalog), is a supernova remnant and pulsar wind nebula. The name – Crab Nebula – is due to the Earl of Rosse, who thought it looked like a crab; it’s not in the constellation Cancer (the Crab), rather Taurus (the Bull).

The supernova which gave rise to the Crab Nebula was seen widely here on Earth in 1054 (and so it’s called SN 1054 by astronomers); it is perhaps the most famous of the historical supernovae. It is certainly one of the brightest (estimated to be –7 at peak), partly because it is so close (only 6,300 light-years away), and partly because it’s not hidden by dust clouds. The expansion of the nebula – as in seen-to-be-getting-bigger, rather than the-gas-is-moving-very-fast – was first confirmed in 1930.

As it was a core collapse supernova (a massive star which ran out of fuel), it left behind a neutron star; by chance, we are in line with its ‘lighthouse beam’, so we see it as a pulsar (all young neutron stars are pulsars, but not all of them have beams which point to us in one part of the cycle). It’s a pretty fast pulsar; the neutron star rotates once every 33 milliseconds. Because it’s so young and so close, the Crab Nebula pulsar was the first to be detected in the visual waveband, and also in x-rays and gamma rays. Being the source of the tremendous output of energy, from both the pulsar wind nebula and the pulsar itself, and as energy is conserved, the pulsar is slowing down, at a rate of 15 microseconds per year.

The inner part of the Crab Nebula, the pulsar wind nebula, contains lots of really hot (‘relativistic’) electrons spiraling around magnetic fields; this creates the eerie blue glow … synchrotron radiation. This makes the Crab Nebula one of the brightest objects in the x-ray and gamma ray region of the electromagnetic spectrum, and as it is a relatively steady source (unlike most high energy objects) it has given its name to a new astronomical unit, the Crab. For example, a new x-ray source may be 2 mCrab (milli-Crab), meaning 0.002 times as strong an x-ray source as the Crab Nebula.

This SEDS page has a lot more information on the Crab Nebula, both historical and contemporary.

Such an intensively studied object, no wonder there are lots of Universe Today stories on it; for example Nearly a Thousand Years After the Death of a Star, Giant Hubble Mosaic of the Crab Nebula, The Peculiar Pulsar in the Crab Nebula, Astronomers Locate High Energy Emissions from the Crab Nebula, and Evidence of Supernovae Found in Ice Core Sample.

Astronomy Cast’s Neutron Stars and Their Exotic Cousins has more on pulsars, and Nebulae more on nebulae.

Sources: Caltech Astronomy, SEDS, Stanford University SLAC

What are WIMPs?

WIMPs are Weakly Interacting Massive Particles, hypothetical particles which may be the main (or only) component of Dark Matter, a form of matter which emits and absorbs no light and which comprises approx 75% of all mass in the observable universe.

The ‘weakly’ is a bit of a pun; WIMPs would interact with themselves and with other forms of mass only through the weak force (and gravity); get it? More plays on words: WIMP, the word, was created after the term MACHO (Massive Astrophysical Compact Halo Object) entered the scientific literature.

WIMPs are massive particles because they are not light; they would have masses considerably greater than the mass of the proton (for example). Being massive, WIMPs would likely be cold; in astrophysics ‘cold’ doesn’t mean ‘below zero’, it means the average speed of the particles is well below c. Neutrinos are weakly interacting particles, but they are not massive, so they cannot be WIMPs (besides, neutrinos aren’t hypothetical, and they’re hot, very hot … they travel at speeds just a teensy bit below c).

Or maybe they are … if there is a kind of neutrino which is really, really massive (a TeV say) then it would certainly be a WIMP! However, the latest results from WMAP seem to rule out this kind of WIMP-as-neutrino.

There are quite a few experiments – active or planned – looking for WIMPs; some Universe Today stories on them are New Proposal to Search for Dark Matter, Searching for Dark Matter Particles Here on Earth, Digging for Dark Matter, the Large Underground Xenon (LUX) Detector, and A Prototype Detector for Dark Matter in the Milky Way. It may turn out that WIMPs remain entirely hypothetical (the particles which comprise Dark Matter may not be WIMPs, for example) or only a minor component of Dark Matter. In the latter case they’d follow MACHOs not only because they were named at a later time … MACHOs have been detected in the Milky Way halo, but there aren’t anywhere near enough of them to account for the rotation curve of our galaxy.

This section of a review by Princeton University’s Dave Spergel is rather old but still one of the best concise technical descriptions; and this Particle Data Group review (PDF file) lists WIMP searches (though it’s a bit dated). NASA’s Imagine the Universe! has a page on Possibilities for Dark Matter that includes a simple overview of WIMPs.

Want to hear, rather than read, more? Check out the Astronomy Cast episode The Search for Dark Matter!

Source: NASA

Ecliptic

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Imagine you could see the position of the Sun, in the sky, relative to the stars (and galaxies, and quasars, and …). If you could, and if you plotted that position throughout the year you’d get a line; that line is called the ecliptic.

And why is it called the ecliptic? Because when the new or full Moon is very close to this, there will be an eclipse (of the Sun, and Moon, respectively).

The Earth goes round the Sun, in an orbit. That orbit defines a plane, which is an infinite two-dimensional sheet; the plane of the ecliptic.

The other planets in the solar system orbit the Sun in planes too, but those planes are slightly tilted with respect to the plane of the ecliptic … so transits of Venus (across the Sun) are quite rare (most times Venus passes either above or below the Sun, when it’s between Earth and the Sun). Mutual transits and occultations of planets are even rarer.

If you’re in a location relatively free of light pollution, on a clear, moonless night you may see zodiacal light. If you trace a line through the middle of it, you’re tracing the ecliptic (zodiacal light is due to reflection of sunlight off dust; dust in the solar system is concentrated in a plane close to the ecliptic plane).

Today astronomers use equatorial coordinates to give positions on the sky, right ascension (RA) and declination (Dec); these are like projections of longitude and latitude out into space (or onto the celestial sphere). However, in Europe ecliptic coordinates were used (up to the 17th century anyway). Here’s a curious fact: historically, Chinese astronomers used equatorial coordinates!

Universe Today stories: Plane of the Ecliptic, Vernal Equinox – Busting the Myth of Balancing Eggs, and Find the Zodiacal Light.

More: Astronomy Cast on Orbit of the Planets, and a Glow After Sunset.

Altazimuth

Altazimuth is a contraction of altitude-azimuth; in astronomy it most often refers to a type of telescope mount (and is sometimes called alt-az), but it can also mean a coordinate system.

Altitude means the angular distance above the horizon; straight up (overhead) is 90o (and is called the zenith). Azimuth is also an angular distance, measured clockwise from north (so east is 90o). Any point, or direction, in the sky has one – and only one – altitude and azimuth; in other words, the altitude and azimuth are the coordinates of the point (on the celestial sphere).

An altazimuth telescope mount is one that can move separately in altitude (up and down, vertically) and azimuth (side to side, horizontally). Small telescopes used by amateur astronomers tend to have altazimuth mounts; larger ones tend to have equatorial mounts … unless they are Dobsonian. Why? Because while alt-az mounts are generally cheaper, tracking astronomical objects (like stars) is much easier with equatorial mounts.

Historically, the telescopes used by professional astronomers did not have alt-az mounts, because automatic tracking was impossible. As computers became powerful and cheap enough, they could be used to control the motors on each axis of an altazimuth mount; today, almost all ground-based astronomical telescopes have altazimuth mounts, whether optical, radio, or even high energy gamma ray! The first really large optical telescope to use an altazimuth mount is the 6-meter Bol’shoi Teleskop Azimultal’nyi, in Russia.

Universe Today’s Telescope Mount, Telescope Parts, Telescope Tripod, and How To Use a Telescope are great resources for learning more.

The Astronomy Cast episode Choosing and Using a Telescope covers the benefits of alt-az mounts (vs equatorial), and Telescopes, the Next Level gives insight into tomorrow’s professional ones.

Black Dwarf

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A black dwarf is a white dwarf that has cooled down to the temperature of the cosmic microwave background, and so is invisible. Unlike red dwarfs, brown dwarfs, and white dwarfs, black dwarfs are entirely hypothetical.

Once a star has evolved to become a white dwarf, it no longer has an internal source of heat, and is shining only because it is still hot. Like something taken from the oven, left alone a white dwarf will cool down until it is the same temperature as its surroundings. Unlike tonight’s dinner, which cools by convection, conduction, and radiation, a white dwarf cools only by radiation.

Because it’s electron degeneracy pressure that stops it from collapsing to become a black hole, a white dwarf is a fantastic conductor of heat (in fact, the physics of Fermi gasses explains the conductivity of both white dwarfs and metals!). How fast a white dwarf cools is thus easy to work out … it depends on only its initial temperature, mass, and composition (most are carbon plus oxygen; some maybe predominantly oxygen, neon and magnesium; others helium). Oh, and as at least part of the core of a white dwarf may crystallize, the cooling curve will have a bit of a bump around then.

The universe is only 13.7 billion years old, so even a white dwarf formed 13 billion years ago (unlikely; the stars which become white dwarfs take a billion years, or so, to do so) it would still have a temperature of a few thousand degrees. The coolest white dwarf observed to date has a temperature of a little less than 3,000 K. A long way to go before it becomes a black dwarf.

Working out how long it would take for a white dwarf to cool to the temperature of the CMB is actually quite tricky. Why? Because there are lots of interesting effects that may be important, effects we cannot model yet. For example, a white dwarf will contain some dark matter, and at least some of that may decay, over timespans of quadrillions of years, generating heat. Perhaps diamonds are not forever (protons too may decay); more heat. And the CMB is getting cooler all the time too, as the universe continues to expand.

Anyway, if we say, arbitrarily, that at 5 K a white dwarf becomes a black dwarf, then it’ll take at least 10^15 years for one to form.

One more thing: no white dwarf is totally alone; some have binary companions, others may wander through a dust cloud … the infalling mass generates heat too, and if enough hydrogen builds up on the surface, it may go off like a hydrogen bomb (that’s what novae are!), warming the white dwarf quite a bit.

More from Universe Today: How Does a Star Die?, Why Do Stars Die?, and Hubble Discovers a Strange Collection of White Dwarf… Dwarfs.

The End of the Universe Part 1 and Part 2 are Astronomy Cast episodes worth listening to, as are The Life of the Sun and The Life of Other Stars.

References:
NASA
NASA: Age of the Universe
Wikipedia

Supernova 1987A

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The first supernova in 1987 (that’s what the “A” means) was the brightest supernova in several centuries (and the first observed since the invention of the telescope), the first (and so far only) one to be detected by its neutrino emissions, and the only one in the LMC (Large Magellanic Cloud) observed directly.

Ian Shelton, then a research assistant with the University of Toronto, working at the university’s Las Campanas station, and Oscar Duhalde, a telescope operator at Las Campanas Observatory, were the first to spot it, on the night of 23/24 February 1987 (around midnight actually); over the next 24 hours several others also independently discovered it.

The IAU’s CBAT went wild! That’s the International Astronomical Union’s Central Bureau for Astronomical Telegrams, the clearing house for astronomers for breaking news. You can read the historic IAUC (C for Circular) 4416 here.

Once the discovery of SN 1987A became known, physicists examined the records from various neutrino detectors … and found three, independent, clear signals of a burst of neutrinos several hours before visual discovery, just as predicted by astrophysical models! Champagne flowed.

Not long afterwards, the star which blew up so spectacularly – the progenitor – was identified as Sanduleak -69° 202a, a blue supergiant. This was not what was expected for a Type II supernova (the models said red supergiants), but an explanation was quickly found (Sanduleak -69° 202a had a lower-than-modelled oxygen abundance, affecting the transparency of its outer envelope).

The iconic Hubble Space Telescope image (above) of SN 1987A shows the inner ring, where the debris from the explosion is colliding with matter expelled from the progenitor about 20,000 years ago; more from the Hubble here.

AAVSO (American Association of Variable Star Observers) has a nice write-up of SN 1987A.

No wonder, then, that SN 1987A features so often in Universe Today stories; for example Supernova Shockwave Slams into Stellar Bubble, XMM-Newton’s View of Supernova 1987A, Supernova Left No Core Behind, and Hubble Sees a Ring of Pearls Around 1987 Supernova.

SN 1987A figures prominently in Astronomy Cast The Search for Neutrinos, and in Nebulae.

References:
AAVSO
University of Oregon

Kepler’s Third Law

Johannes Kepler

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“The square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit” That’s Kepler’s third law. In other words, if you square the ‘year’ of each planet, and divide it by the cube of its distance to the Sun, you get the same number, for all planets.

(The other two are “the orbit of each planet is an ellipse with the Sun at a focus”, and “a line between a planet and the Sun sweeps out equal areas in equal times”.)

Copernicus, Kepler, and Newton dealt a one-two-three knockout blow to the idea – thousands of years old – that the Sun (and planets) moved around the Earth. Copernicus put the Sun at the center, Kepler modified Copernicus’ circular motions (and provided a simple, quantitative description of the actual motion), and Newton explained how it all worked (gravity).

Kepler worked out his three laws from detailed records of observations of the positions of the planets (known at the time, Mercury, Venus, Mars, Jupiter, and Saturn) – especially Mars – painstakingly compiled by Tycho Brahe.

Kepler’s third law (in fact, all three) works not only for the planets in our solar system, but also for the moons of all planets, dwarf planets and asteroids, satellites going round the Earth, etc. Well, not quite; if the secondary body – a planet, say – has a mass that’s a significant fraction of the primary one (the Sun, say), then the law needs a small tweak.

By showing how Kepler’s laws could be derived from his universal law of gravitation, Newton united heaven and earth, perhaps the greatest revolution in science (OK, Darwin’s revolution may be greater). Before Newton, the heavens were thought to work according to rules quite different from the ones which governed things on Earth.

NASA’s Imagine the Universe! has a neat demonstration of Kepler’s laws, and this PDF file (from the University of Tennessee Knoxville’s Maths Department) gives a simple derivation of Kepler’s laws, from Newton’s universal law of gravitation.

Universe Today articles with more information: Kepler’s Laws, Let’s Study Law: Kepler Would Be So Proud, and Happy Birthday Johannes Kepler.

Gravity, an Astronomy Cast episode, also discusses Kepler’s third law, as does Where is the Center of the Universe?.

Abiogenesis

What are Fossils

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How did life on Earth arise? Scientific efforts to answer that question are called abiogenesis. More formally, abiogenesis is a theory, or set of theories, concerning how life on Earth began (but excluding panspermia).

Note that while abiogenesis and evolution are related, they are distinct (evolution says nothing about how life began; abiogenesis says nothing about how life evolves).

Intensive study of the Earth’s rocks has turned up lots and lots of evidence that some kinds of prokaryotes lived happily on Earth about 3.5 billion years ago (and there’re also pointers to the existence of life on Earth in the oldest rocks). So, if life arose on Earth, it did so from the chemicals in the water, air, and rocks of the early Earth … and in no more than a few hundred million years.

Because there are no sedimentary rocks older than about 3.7 billion years (and no metamorphic ones older than about 3.9 billion years), and because the oldest such rocks already contain evidence that there was life on Earth then, testing abiogenesis theories must be done by means other than geological.

There is a long history of attempts to create various organic molecules – such as amino acids – from simple precursors such as carbon dioxide, ammonia, and water, in conditions which simulate those of the early Earth. Those of Miller and Urey, in 1953, are the most famous (and the first).

It turns out that it’s pretty easy to form many kinds of organic molecules, in a wide range of environments … so the focus of research today is on how life could arise from any particular brew. And the hard part is how reliable self-replication get going (if you can make some sort of primitive cell in a test tube, it isn’t a form of life if it can’t reproduce itself!). So far, it seems that RNA and DNA cannot have been involved (too hard to form and stay stable), but several simpler kinds of molecules may work.

Well, that’s one hard part; another is how can a stable bag of chemicals form? (There have been some exciting recent discoveries which may help answer at least part of this question).

A different approach – than reproduction – to finding the key to how life got started involves asking how metabolism arose; how can a bag of chemicals take in ‘food’, process it (to supply energy to all the other chemical processes going on in the bag), and get rid of the waste?

The TalkOrigins website has a summary of abiogenesis, though it is now somewhat dated (much has happened in just the last three years)!

Abiogenesis in its strict sense (origin of life on Earth) is a bit off the track for Universe Today; however, conditions under which life might spontaneously arise, on other planets (etc) is not. Some Universe Today stories on this are Sub-surface Oceans In Comets Suggest Possible Origin of Life, Add Heat, Then Tectonics: Narrowing the Hunt for Life in Space, and Has Liquid Water Been Detected on Mars?

Hawking Radiation

Zero Gravity Flight

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When humans starve, they grow thin and eventually die; when a black hole starves, it too grows thin and dies … but it does so very spectacularly, in a burst of Hawking radiation.

At least that’s the way we understand it today (no black-hole-pining-away has yet been observed), and the theory may be wrong too.

Cosmologist, astrophysicist, and physicist Stephen Hawking showed, in 1974, that black holes should emit electromagnetic radiation with a black body spectrum; this process is also called black hole evaporation. In brief, this theoretical process works like this: particle-antiparticle pairs are constantly being produced and rapidly disappear (through annihilation); these pairs are virtual pairs, and their existence (if something virtual can be said to exist!) is a certain consequence of the Uncertainty Principle. Normally, we don’t ever see either the particle or antiparticle of these pairs, and only know of their existence through effects like the Casimir effect. However, if one such virtual pair pops into existence near the event horizon of a black hole, one may cross it while the other escapes; and the black hole thus loses mass. A long way away from the event horizon, this looks just like black body radiation.

It turns out that the smaller the mass a black hole has, the faster it will lose mass due to Hawking radiation; right at the end, the black hole disappears in an intense burst of gamma radiation (because the black hole’s temperature rises as it gets smaller). We won’t see any of the black holes in the Milky Way explode any time soon though … not only are they likely still gaining mass (from the cosmic microwave background, at least), but a one sol black hole would take over 10^67 years to evaporate (the universe is only 13 billion years old)!

There are many puzzles concerning black holes and Hawking radiation; for example, black hole evaporation via Hawking radiation seems to mean information is lost forever. The root cause of these puzzles is that quantum mechanics and General Relativity – the two most successful theories in physics, period – are incompatible, and we have no experiments or observations to help us work out how to resolve this incompatibility.

Colorado University’s Andrew Hamilton has a good introduction to this topic, as does Usenet Physics FAQ (often recognized by John Baez’ association with it).

Some Universe Today stories which include Hawking radiation are Synthetic Black Hole Event Horizon Created in UK Laboratory, How to Escape from a Black Hole, and When Black Holes Explode: Measuring the Emission from the Fifth Dimension.

Black Holes Big and Small, and The Large Hadron Collider and the Search for the Higgs-Boson are two Astronomy Casts relevant to Hawking radiation.

Sources:
Colorado University
ThinkQuest
University of California – Riverside