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Your best chance to see Mercury March 30, 2010

Posted by Peter Hornby in astronomy.
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The planet Mercury, the closest planet in our solar system to the Sun, is not an easy object to see.  It’s always close to the Sun in the sky, so much so that you can pretty much never see it against a dark sky background, and it’s a small object, compared to the other major planets, so it never gets as bright as Jupiter or Venus.

The next couple of weeks offers as good a chance to catch Mercury as you’ll ever find, for northern hemisphere observers at least.  It’s fairly distant (relatively speaking) from the Sun in the evening sky, and, more importantly, Venus is in the same region of sky.  You can’t miss Venus, always the brightest object in the sky other than the Sun and the Moon, so it’s easy to use Venus as a guide to spot Mercury.

Check out this article from Sky & Telescope’s website.  Study the graphic, find the best western horizon you can and head out soon after sunset (around 7:15pm in southern California).  You’ll have a reasonable shot any night between now and early next week, after which Mercury will get too faint to be easily seen.  Your best best will probably be Saturday.

Good luck!

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The centre of the galaxy – as never before March 24, 2010

Posted by Peter Hornby in astronomy.
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Here’s a remarkable movie loop, prepared by Professor Andrea Ghez and her research team at UCLA, and presented at a public lecture following her recent Aspen Center for Physics conference on “The Formation and Evolution of Black Holes”.

Let’s look at what’s going on here.  We’re looking at a tiny, tiny area, no more than a second of arc across, at the exact centre of the Milky Way galaxy, roughly 25,000 light years away.  We’re pretty certain that, like many, if not all, other galaxies, our Milky Way has a huge black hole at its centre.  One way astronomers can probe the location, size and habits of the central black hole is by watching the orbits of objects close to it.  Professor Ghez’s animation shows the movement of some of these objects over the fifteen year period from 1995 to 2010.  It’s made from real, actual images, taken using telescopes such as the Keck 10-meter monsters on Mauna Kea, and using the most sophisticated adaptive optics techniques imaginable.  It’s an astonishing feat of observational astronomy.

What we see is that these objects are orbiting something – you can see that two of them have completed, or almost completed, full orbits over the fifteen years spanned by the movie.  The object (S0-16) coming in from upper left, taking a fast turn around the black hole, and heading out again, approaches as close as 45 astrononomical units (1 AU is the distance of the earth from the Sun), which, to give you some scale, is around four billion miles, not much larger than the distance of Pluto from the Sun,  At its closest approach, it’s moving at 4% of the speed of light!  These orbits give a direct estimate of the mass of the central black hole – it comes out as roughly 3.7 million solar masses.  The current limiting factor on this estimate is that we don’t know exactly how far away the centre of the galaxy actually is!

Nowhere in the galaxy are conditions anything like as extreme as we’re seeing here.  The gravitational gradients are huge.  Professor Ghez has suggested, according to a commenter at the Cosmic Variance blog entry linked below, that it should actually be possible, within a decade or so, to directly observe the precession of the orbits of these objects which is predicted by General Relativity.

The idea that we can directly watch stars orbiting a black hole at the centre of our galaxy defies belief.

Link from Daniel Holz at Cosmic Variance.

Acknowledgement : This animation was created by Prof. Andrea Ghez and her research team at UCLA and are from data sets obtained with the W. M. Keck Telescopes.  Other images and animations can be seen at Prof. Ghez’s group website

The Big Picture at Saturn October 20, 2009

Posted by Peter Hornby in astronomy.
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You’re familiar with “The Big Picture“, right?  Three days a week, Alan Taylor, a developer for the Boston Globe’s website, publishes a set of high-res photographs of, well, almost anything interesting which comes across his desk – the last couple of entries have looked at World Animal Day 2009 and the World Gymnastics Championships in London.  Pretty much every post is worth looking at – Taylor’s approach of providing huge, beautiful, high-quality images makes for compelling viewing.

And now he’s left Earth entirely.  The most recent post is a set of shots of Saturn captured by the Cassini spacecraft, and they’re some of the most incredible images I’ve ever seen.  Just keep in mind what you’re looking at, and how these images were created, and, if you’re like me, you’ll be lost for superlatives.

If you want more, keep an eye on the Planetary Society’s blog, run by Emily Lakdawalla.  Emily keeps track of pretty much everything going on in the world of Solar System exploration – Messenger at Mercury, Cassini at Saturn, Spirit and Opportunity on Mars, and it’s well worthwhile taking a look every now and then.

And, finally, this is not the first time The Big Picture has showcased Cassini.  Take a look at this set, published in May 2008 and representing a selection of the best images returned by Cassini over the four years (now five and still going strong) of its mission.  Just jawdroppingly amazing!

The most luminous event in the universe May 19, 2008

Posted by Peter Hornby in astronomy.
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Something astonishing happened in the sky earlier this year, something unique in human experience. I’d like to talk a little about what this event was, and why it was so remarkable.

Let’s start with some background.  Since the late sixties, astronomers have been recording, analyzing – and puzzling over – enigmatic objects called gamma ray bursts.  Gamma rays are the most energetic form of electromagnetic radiation, so when we see something shining at gamma ray wavelengths, we know that something extraordinarily powerful is behind it. The gamma ray burst (GRB) is a flash of gamma rays, typically lasting no more than a few seconds, coming from a place in the sky where nothing had previously been recorded. As they fade, GRBs normally show an afterglow at X-ray, ultraviolet or visible wavelengths.

For decades, no visible light counterpart – a star or galaxy – could ever be found, but the suspicion was that, since the distribution of GRBs on the sky showed no relation to the structure of our galaxy, they must be extragalactic, and consequently extraordinarily luminous. This suspicion was confirmed in 1997 when a faint galaxy was found at the location of a GRB.

We’re now in the midst of a revolution in GRB study, dating from the launch of the SWIFT orbiting observatory in late 2004. SWIFT combines a sensitive gamma ray detector with the ability to rapidly slew its other instruments to the location of a burst within seconds. Not only that, there’s now an amazing network which permits co-operating ground-based observatories, both professional and amateur, to be activated, totally automatically, within seconds of SWIFT’s detection of a GRB.

So why I am I so excited?  Well, the July “Sky and Telescope” magazine arrived this morning, with a brief analysis of a March 19 GRB detected by SWIFT. What’s totally amazing is that the visible-light afterglow of this event was, for a few seconds, bright enough to be visible to the unaided eye.  This struck me as the most astonishing event and I threw a few calculations together to get a sense of just how unbelievable it really was – what’s actually meant when we describe this event as the most luminous single event ever seen, anywhere in the universe.

So, let’s start with some numbers. The March 19 event, called GRB 080319B, occurred in a galaxy 7.5 billion light years away. The galaxy itself can only be seen by the most rigorous imaging techniques. By contrast, the galaxies we can see with our backyard telescopes are only tens of millions light years away – the famous Andromeda galaxy (M31) is just over 2 million light years away and it’s about 60 million light years to the huge Virgo cluster of galaxies, a favourite telescopic target for amateur astronomers in the spring sky.

That’s million, by the way, not billion. At 7.5 billion light years, GRB 080319 B was a long, long way away.

So, how bright was it?

Let’s digress a bit to discuss the way astronomers talk about the brightness of celestial objects. The key thing to realise is that how bright an object appears to us in the sky is no guide to its intrinsic luminosity. Is it a dim candle close by, or a distant searchlight? Astronomers use the apparent magnitude scale to describe how bright an object looks to us. How apparent magnitude is defined doesn’t really matter here. It’s enough to understand that bigger numbers mean fainter objects, on a scale where the faintest star visible to the unaided eye is about magnitude 6, the brightest stars in the sky are around magnitude -1, Venus is around -4, the full moon around -12 and the sun around -26.

Following our analogy, if the candle were ten feet away and the searchlight ten miles, they might have the same apparent magnitude. However, if you move them so that they’re at the same distance from you, it becomes immediately obvious that the searchlight is vastly more powerful than the candle. The absolute magnitude, which astronomers use to describe the intrinsic brightness of an object, is defined in an analogous way. It’s just the apparent magnitude an object would have if you moved it to a standard distance – in this case 32.6 light years.

Let’s look at the Sun as an example. It’s so bright because it’s so close. When you compare it to other stars, though, is it a candle or a searchlight? The answer is – it’s a candle. Move the Sun out to 32.6 light years, and it becomes a dim yellow star of magnitude 4.8, barely visible to the naked eye. By contrast, Rigel, the brilliant white star in the lower right of the famous Orion figure, is a distant celestial searchlight. Even at a distance of 700-800 light years, it appears as one of the brightest stars in our sky, with an apparent magnitude of 0.1. Move it in to 32.6 light years, and it dazzles at magnitude -7.0, ten times brighter even than Venus.

If we go a little crazy, and try to estimate the absolute magnitude of an entire galaxy, we come out with, as you’d expect , some very negative numbers. The absolute magnitude of our own Milky Way – the combined luminosity of its 200 billion or so stars – is estimated at -21.

The estimated absolute magnitude of GRB 080319B was -34.

I still can’t quite believe this.  GRB 080319B was a single object – probably a massive star collapsing catastrophically into a black hole – which was, for its few final moments of life, a million times brighter than the entire galaxy it lived in, bright enough to be visible to an unaided human eye halfway across the universe.

Let me conclude with a couple of broad comparisons and a pen picture.

  • Place it anywhere at all in our galaxy. GRB 080319B would have appeared as a point source way, way brighter than the full moon. Anywhere within 2000 light years, it would have appeared as bright as the Sun.
  • Place it in the Andromeda Galaxy, an association of several hundred billion stars 2 million light years from us, and visible as a faint naked eye object, around magnitude 4. GRB 080319B would have appeared around magnitude -10, still almost as bright as the full moon.

Finally, I want you to imagine that you’re observing the galaxy M58, 60 million light years away, with your backyard telescope. It’s a dark, steady, moonless night, and you’re concentrating on a faint, elliptical haze, trying to detect the spiral structure you know is there. Suddenly, a point of light appears. In a few seconds, it’s so bright that looking at it through your telescope hurts your eyes. You step away from the telescope and look up. The object continues to brighten, until it outshines Venus. A few seconds more and it starts to fade, and within a couple of minutes, it’s lost to your sight.

We’ve only been studying gamma ray bursts seriously for a few years. Maybe, one day, some lucky amateur astronomer will be in the right place, at the right time, to make the observation of a lifetime, to see, with his own eyes, the brightest object anywhere in the universe.

One last note. Sky and Telescope notes that GRB 080319B occurred seven hours after the death of the revered science-fiction writer and visionary Arthur C. Clarke.  Some astronomers are already referring to GRB 080319B as the “Clarke Burst”, the universe’s spectacular sendoff to one of our own brightest stars.

Celestial mechanics works July 25, 2007

Posted by Peter Hornby in astronomy.
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The Planetary Society blog, mostly written and researched by the seemingly indefatigable Emily Lakdawalla, is an outstanding resource if you’re interested in what’s happening in our exploration of the Solar System.  Emily seems to have everyone in the field of solar system research on her speed-dial list, and serves up the latest authoritative information from all the current and future projects and missions.

I was browsing the blog the other day when I ran across a remarkable image.  The image was in a post with the arresting title of “Hey, Moon! Get out of the way of Cassini”, and the point was to demonstrate, in a very graphic way, that this was a situation where our own moon almost got in the way of Cassini’s communication with Earth. The image was generated by a wonderful Solar System simulator, written by David Seal at JPL, which I hadn’t previously been aware of.  You can reproduce the image by going to the simulator and asking for the view of Earth as seen from Cassini on 2007 July 16 at 22:00 UTC. 

Understandably, Emily’s perspective in her blog entry was that of a space scientist – the pesky moon is obstructing Cassini’s line of sight to Earth.  It struck me, though, that if the moon was transiting Earth as seen from Cassini at Saturn, the same celestial geometry should show up on Earth as an occultation of Saturn.  And indeed it did.  Here’s an image showing the track of the occultation.  It turns out that most of the track was over the Pacific in daylight, which means that it wouldn’t have helped the scientists at Goldstone to look out of the window.  The image shows the occultation track crossing the South American coast around sunset, shortly after which the Moon and Saturn set.

I can’t quite put my finger on why this gives me a sense of comfort, but it does.  Maybe it’s just good to feel reassured that celestial mechanics works.