Ask an astronomer

This afternoon I was the astronomer in an 'ask an astronomer' session at the observatory. This basically involved me talking about radio telescopes, the universe and everything for about an hour. I also answered lots of questions in that time.

As always, one of the best questions came from a tiny little kid. He wanted to know how gas giant planets were formed. I tried my best to explain about the best theories of the creation of the solar system; the nebula hypothesis explains why the heavy stuff collapsed into the middle - creating the inner, rocky planets - and the gas was mainly left in the outer solar system. I also got a question from an older gentleman who wanted me to justify astronomy. It is a question that comes up quite frequently and I try to answer it on several different levels. I always start out by saying that any civilized society should try to find out about the universe around it. That is part of what makes us civilized. I then moved on to the tests of physics, specifically gravity and relativity which do have practical consequences. I finished up on technological development from detectors/receivers (optical, radio, x-ray, infrarecd etc) which have lots of practical applications. In many ways it is a very important question as we should justify to the public why they fund us. I must admit that I'm in it for the sake of the knowledge though.

Now I have to come clean; I made at least one mistake in my hour. At one point, I accidentally claimed that the Sun would explode as a supernova in 5 billion years time. Unfortunately, this isn't true, as it will shed its outer layers relatively slowly (by astronomical standards) and end up producing a planetary nebula. I know this, I really do, but in the hour of rambling conversation, I got confused and was thinking of stars ten times the mass of the Sun. Sometimes astronomers make mistakes.

Posted in astro blog by Stuart on Thursday 31st Mar 2005 (16:11 UTC) | Permalink

Manchester Light Pollution

Tonight, I went observing with Tony and Megan. We used the 10" Meade on the roof of the physics department in the city centre. To be honest, the light pollution in Manchester is really bad, so seeing all but the brightest objects by eye can be a challenge. Considering that the conditions were far from perfect, we did manage to image a few objects although I'm still not convinced about NGC 884. To illustrate the point about light pollution, I thought I would include a picture that I took with my digital camera.

Light pollution in Manchester

Posted in astro blog by Stuart on Tuesday 29th Mar 2005 (02:17 UTC) | Permalink

Crater or mountain?

Lunar craterLunar crater

What do you see when you look at the two pictures above? To some people, the image on the left looks as though it contains domed mountains whereas the one of the right looks like it is covered in craters. Actually, both the images are of the same landscape but the one on the left has been turned upside down. What you are seeing is an optical illusion caused by our perception of distance. In pictures taken 'the right way up', the bottom is usually nearer and the top is further away but for the image on the left, the top is nearer and the bottom further away. This plays tricks on the brain causing it to incorrectly interpret what it is seeing. The effect has been causing some people to get very confused when looking at images taken by ESAs SMART-1. Note: these images aren't actually from SMART-1!

Posted in astro blog by Stuart on Sunday 27th Mar 2005 (21:57 UTC) | Permalink

Rian, there's your belt!

I wrote some time ago about a few of my experiences with an inflatable planetarium that we have been taking around schools in the Cheshire/Manchester area. Overall it has been great fun as most of the kids that go into it are amazed and nearly every class goes "oooh, aaahhhh, wow!" when it first goes dark and they see the stars. I would say from their responses that most of them have never seen so many stars in their lives. One of the by-products of these visits, is a collection of things that kids - and teachers - say inside the planetarium. So, in the style of Mr Astronomy Guy, I though I would share a couple.

The inflatable planetarium looks like a big silver igloo with an airlock entrance at the front and a fan at the back blowing air inside. The airlock is what keeps the higher air pressure inside the dome and thus keeps it inflated when the show is going. However, calling it an airlock prompts questions such as "do you float when you go in there?". This highlights the misconception that lack of air equates with zero-gravity conditions. I blame Hollywood for that one. At another show, pointing out Orion's belt prompted one kid to shout to his neighbour (yes shout!): "Rian, there's your belt!". And finally, while talking about the discovery of planets outside the solar system and the possibility of alien life, one small boy said, in all seriousness, "I've met an alien.......my sister".

Posted in astro blog by Stuart on Sunday 27th Mar 2005 (20:51 UTC) | Permalink

Spring forward

In the early hours of tomorrow morning, at 1am, the clocks go forward by one hour in the UK. Rather than use GMT (or UT), we will be using British Summer Time (BST). If you're like me, you probably never remembered when the clocks were going to change and always found out about it the night before - if you were lucky. Part of the reason that we don't remember about the change is that it doesn't occur on a fixed calendar date. I had always assumed that there must be a rule similar to the one for Easter, but nobody I knew ever seemed to know what it was. Well there is no need to feel bad about ignorance of the rules as it seems that there have been several different ones over the years.

Summer time was first defined in an Act of 1916 and occurred each year up until the Second World War. During the war, the UK was on double summer time, meaning that during the winter we were one hour ahead of GMT and then two hours ahead during the summer. After the war we went back to summer time only until 1968, when we got stuck one hour ahead of GMT for three years. From 1972 onwards, we were back to GMT and summer time, although the exact time of change each year was redefined a few times. From 1995 to 1997 the rules were more or less the same as the Seventh EU Directive. Finally, in 1998, the EU adopted The Ninth European Parliament and Council Directive on Summer Time Arrangements. These arrangements state that summer time is kept from the last Sunday in March to the last Sunday in October and the change occurs at 01.00 GMT.

In the past 100 years there have been over seventy pieces of legislation as politicians try to legislate when the sun rises and sets. Although the idea of using 'European time' keeps coming up, it looks as though the rules will stay fixed for a few years at least.

So, if you live in the UK, remember to put your clocks forward one hour before you go to bed.

Posted in astro blog by Stuart on Saturday 26th Mar 2005 (22:17 UTC) | Permalink

Deep Pan Earth

Have you ever thought that the Earth looked like a pizza? It never crossed my mind until now. In a new image, created by Dr. Dirk Petry of NASA Goddard Space Flight Center, the Earth does vaguely resemble a deep pan pizza with extra green peppers and tomato. The image shows us what the Earth would look like if you could see gamma rays - light that has billions of times more energy that the light we normally see with. It was created using seven years worth of data from the Compton Gamma-Ray Observatory. The red image corresponds to gamma rays at 35-100 MeV (mega electron volts); the green image corresponds to gamma rays at 100 MeV to 1 GeV; the blue image corresponds to gamma rays at 1-10 GeV; and the brownish image shows the full mix of energies. The satellite was at an altitude of about 420 km so the image has an extreme wide-angle view.

Gamma Ray Earth

Image credit: NASA/CGRO/EGRET/ Dirk Petry

Posted in astro blog by Stuart on Saturday 26th Mar 2005 (00:47 UTC) | Permalink

Email into space

I just stumbled across a website that lets you talk to aliens! In fact they don't stop there as you can even send emails to little green men if you don't have a phone. The website is called talktoaliens.com and seems to be one of several companies offering to send phone calls, emails and even entire websites into space. On the 11th March a company called Deep Space Communications Network (a bit too similar to NASA's Deep Space Network in name if you ask me) sent a community website advertising classified listings for jobs and housing into space. Do they think that E.T. would like to rent a two bedroom apartment or get a job as an electrician?

Actually, the messages probably won't get very far anyway. The talktoaliens.com dish is only 10.5 feet in diameter so isn't going to be too great. The CNN article claims that messages may go as far as one to three light years so it won't even reach the nearest star outside the solar system. So much for sending a message to 'the deepest reaches of the cosmos' as talktoaliens.com claims. Paying USD 3.99 per minute to talk to empty space reminds me of dodgy star naming companies and their money making schemes.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (22:39 UTC) | Permalink

Seeing an extrasolar planet

When going into schools, I sometimes ask kids if they know the difference between a planet and a star. This can be a tricky question, but some of them remember that a star gives out its own light whereas a planet doesn't; it just reflects light from a star. Well actually, that isn't quite correct as anything with a temperature above -273 Celsius will give out radiation from radio waves to infra-red.

This week, a paper was published in Nature showing that light had been detected from two planets going around stars other than the Sun. Although the European Southern Observatory's VLT imaged what might have been a giant planet last September, this is the first time light has been directly detected from known planets. The observations were made with the Spitzer Space Telescope which directly observed the warm infra-red glows of two previously detected 'hot Jupiter' planets. The planets are named HD 209458b and TrES-1 (don't you just love the names that astronomers come up with) and the latter was first found by two small aperture (10cm) telescopes last August.

The Spitzer detection technique relies on the fact that the orbits of these planets are suitably aligned so that they pass in front of (transit) and behind (occultation) their stars. The infra-red light was monitored as the planet orbited its parent star. Comparing the difference in infra-red measured when the planet was out of view to when it was visible, it was possible to calculate the contribution from the planet alone. Their temperatures have been calculated as over 800 degrees Celsius, so they deserve the name 'hot Jupiters'.

Planets around other stars - extrasolar planets as they are known - were first discovered in 1995. The last decade has seen a string of discoveries bringing the total to 152 planets in 134 separate planetary systems. So next time somebody asks you how many planets there are, you can tell them that there are certainly more than nine.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (22:07 UTC) | Permalink

Dark Energy

Talking of AstronomyNow, that reminds me of another story about dark energy in the local universe. What is dark energy? That's a good question. Basically, nobody knows but most astronomers think it is there. No, honestly. In fact it looks as though it actually makes up more than two thirds of the stuff in the universe.

A few years ago, astronomers thought that the universe was made up of matter (what you, me and this computer are made of) and dark matter. Hang on, dark matter? Oh yeah, dark matter is a form of matter that is 'invisible'; it doesn't interact with light but it still somehow knows about gravity. Dark matter is indirectly 'observed' via the way it makes galaxies rotate. There have been lots of suggestions for what particles could be the dark matter such as neutrinos with mass, weakly interacting massive particles (WIMPS), black holes, massive astrophysical compact halo objects (MACHOS)... the list goes on. So far nobody knows which if any it is, but there are lots of people searching.

So back to dark energy. As hopefully you know, gravity is an attractive force which means that it likes to pull the matter together. We know that the universe is expanding and gravity acts to try to slow it down. However, in recent years, observations seem to have shown that the universe started to speed up again about 5 billion years ago. It is this speed up that is attributed to dark energy. Another good question is why would it choose then to kick in. Again, nobody knows the answer. This sort of thing can keep astronomers awake at night if they aren't awake anyway.

So, the current best guess for the ingredient list of the universe would go something like: matter (baryons) 5%, dark matter (anyone's guess) 25% and dark energy (we're getting desperate here) 70%.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (12:17 UTC) | Permalink

Slacker Astronomy

I've been listening to the latest Slacker Astronomer podcast. It was good to see that they picked up on the same story that I noticed on Astronomy Now this week. The story was headlined "Early universe looks like 'vegetable soup'" but as far as I (or they) can see, the press release doesn't go on to explain itself. In fact, it goes on to describe the universe as a zoo. Sort out your analogies please. Coming up soon, they promise us that they have an interview with Phil Plait of BadAstronomy.com as well as a sound tour of the VLA. Keep it coming guys.

Posted in astro blog by Stuart on Friday 25th Mar 2005 (01:12 UTC) | Permalink

Neutron Star

Once a very large star reaches the end of its life, it swells up as a red giant. Eventually, it converts all its hydrogen to helium and then helium into heavier elements. It quickly runs out of fuel and explodes as a supernova (if it is is massive enough). The centre of the star then collapses down to form a new star that is so dense that protons and electrons get combined and only neutrons can exist - at least in the middle - so it gets called a neutron star. This collapse can also spin it up and if can turn it into a pulsar. While ambling around the Jodrell website I stumbled across this cosmic confectionary wrapper.

Posted in astro blog by Stuart on Thursday 24th Mar 2005 (08:32 UTC) | Permalink

NGC 7027

This beautiful image was taken by the NICMOS instrument onboard the Hubble Space Telescope in 1998. It shows an object named NGC 7027 - a planetary nebula - which is about 15 arcseconds in diameter (about a 120th of the angular diameter of the Moon) that can be found in the direction of Cygnus the Swan.



Despite the name, planetary nebulae are not actually planets. They occur during the death throes of a star as it expels gas and dust outwards. Objects like this acquired the name planetary nebula because they looked a bit like planets when viewed through early telescopes. Although it appears to be very small, the nebula is actually about 14,000 times larger than the distance from the Sun to the Earth, so the entire Solar System could easily be swallowed up inside it.

This image isn't quite what you would see through an optical telescope as it is actually an image made from looking at specific parts of the infrared spectrum (1.10, 2.12, and 2.15 microns wavelength). This is a pseudo-colour image with the shorter wavelength represented as blue and the longest as red. The result is that the different colours tell us about different elements in the nebula. Red shows up regions where molecular hydrogen is being split into individual atoms by UV radiation from the central star. The whiter region in the centre is gas at a temperature of tens of thousands of degrees.

NGC 7027 doesn't just give off light in the optical and infrared parts of the spectrum; it also gives off radio wavesas well. At frequencies around 30 GHz, it turns out to be quite bright (5.45 Jy) and also very stable. This means that it gets used to calibrate measurements at radio frequencies.

Posted in astro blog by Stuart on Saturday 19th Mar 2005 (18:35 UTC) | Permalink

Deep Impact

On July 4th at 06:00 UT, part of the spacecraft Deep Impact will collide with comet 9P/Tempel 1. The date isn't really a coincidence; NASA plan to have a large fireworks show for the US 4th July celebrations. Actually, before you start thinking that we will be hit by debris, don't worry; the comet is a long way from Earth. As it only reaches magnitude 9.7 before impact, I guess that you would still need a telescope to see it even if it brightens by a few magnitudes.

Unfortunately, those of us in the UK will be on the wrong side of the planet to get a good view with our telescopes. The comet will appear close to the star Spica, in the constellation Virgo, so it won't rise until around 16:00 UT. As sunset isn't until around 20:40, it means that we will get our first glimpse at least 14 hours after impact. Luckily, we have the Internet so that we can see live video and other people's pictures.

Posted in astro blog by Stuart on Friday 18th Mar 2005 (00:43 UTC) | Permalink

Spaced Out

It was nice to see that Spaced Out, a UK wide scale model of the Solar System made the Online section of the Guardian today. The project had its original soft launch at Jodrell Bank during Science Week last year. It was only last Friday - ironically a windy and overcast day - that the 'Sun' was officially opened by Sir Francis Graham Smith (13th Astronomer Royal) and some school children from Sacred Hearts Primary School in Oldham. On Tuesday, Megan got to be the 'official astronomer' at the opening of Mercury at Hermitage Primary School in Holmes Chapel. Although Saturn, Uranus and some asteroids have already been opened, the Earth will be launched (!) tomorrow morning in Macclesfield.

With Pluto in Aberdeen, this should be the world's largest scale model of the Solar System at a scale of 1:15 million. That makes it quite a bit bigger than either this one or this one. If you were to place the nearest star (other than the Sun of course!) on the same scale, it would have to be beyond the Moon. As they say, space is big...

Posted in astro blog by Stuart on Thursday 17th Mar 2005 (18:27 UTC) | Permalink

Size limit on stars

According to Donald Figer (Space Telescope Science Institute) in collaboration with Francisco Najarro (Instituto de Estructura de la Materia in Madrid), there is a limit which stops stars being more than about 150 times more massive than our Sun. Donald measured the masses of stars in the Arches cluster, the densest known cluster of stars in our galaxy, using the Near Infrared Camera and Multi-Object Spectrometer onboard the Hubble Space Telescope. Using Francisco's detailed models, they found that there were no stars with masses greater than about 130 times the mass of the Sun, although this doesn't rule out very high mass stars which could be created during stellar mergers. However, some of the apparently high mass stars could actually be binary stars that we can't resolve. This would actually reduce the upper limit. The research was published in Nature.

Posted in astro blog by Stuart on Thursday 17th Mar 2005 (10:38 UTC) | Permalink

Pioneer V

On this day in 1960, the Lovell telescope set a distance record for making contact with a spacecraft. It made contact with the American Pioneer V satellite at a distance of 655,000 km (407,000 miles). That is a bit less than twice the distance of the Moon, so in today's terms in seems quite nearby. Pioneer V was designed to provide the first map of the interplanetary magnetic field between Venus and Earth. It functioned for a record 106 days and the last communication with Earth was from a record distance of 36.2 million km (22.5 million miles). This was beaten a couple of years later by Mariner 2.

Posted in astro blog by Stuart on Monday 14th Mar 2005 (10:34 UTC) | Permalink

Wandering Saturn

I did want to see Mercury yesterday, but unfortunately it was cloudy around sunset. One planet I have seen a lot of recently, in the night sky, has been Saturn. It is really easy to spot right now as it is up in the evening so you don't have to stay up all night. So, for those of you who don't know where it is, here are some directions.

Go outside on a clear evening and, if you're in the northern hemisphere, look towards the south. If you don't know which direction south is, try to remember where the Sun set earlier on and come around to the left of that. What you want to find first is Orion (the Hunter) who can be spotted by looking for the three stars that make up his belt. I have included a screenshot (from Stellarium) to make things a bit easier although I haven't put on any constellation lines as there aren't any in the real sky. Once you've found the belt, you should move in a line up and left through the top left shoulder (the red giant star Betelgeuse) towards three bright objects. Two of these are Castor and Pollux, which form the heads of Gemini (the Twins). The lower of the three objects is the planet Saturn.

Finding chart for Saturn

During a single night, planets seem to move from east to west along with the stars. If you follow them from night to night you will see that they actually travel west to east as they orbit the Sun. This is what Saturn was doing until November last year when it made a straight line with Castor and Pollux. Since then it has been moving backwards (east to west) in what is known as retrograd motion. In a few weeks, Saturn will come to a stop (relative to the stars) and once again begin to move towards the east. This strange dance on the sky, is the reason planets are called planets; it comes from the Greek meaning 'wandering star'. The strange motion of the planets caused confusion for thousands of years because people thought that the planets, Sun and stars orbited the Earth. It was finally Nicolaus Copernicus who came up with a neat mathematical model to explain the motions. This required the Earth and the planets to be orbiting the Sun and so the Copernican model of the Solar System was created.

Posted in astro blog by Stuart on Sunday 13th Mar 2005 (14:16 UTC) | Permalink

Mercury setting

As Ian Morison and Phil Plait both mention, today is a great day to see the planet Mercury. Mercury is normally a difficult planet to see as it only ever gets as far as 18 degrees from the Sun as seen from the Earth. Tonight, as the Sun sets, you should be able to spot Mercury around 15 degrees above the horizon. It may be difficult to see at first, as the sky is still bright (see screenshot from the excellent Stellarium below), but as it gets darker you should be able to make it out with the unaided eye. The only thing you need to be careful about is to pick a spot with an unobstructed western horizon. Oh, and if you go out before the sun sets, remember not to look directly at the Sun; I don't want you all going blind!

Mercury shortly after sunset

Posted in astro blog by Stuart on Saturday 12th Mar 2005 (14:11 UTC) | Permalink

California Nebula

The California Nebula, or NGC 1499, is the subject of today's astronomy picture of the day. If you squint you can just about convince yourself that it looks a bit California-shaped. Not as easy to see as the Sombrero Galaxy or the Horsehead Nebula in my opinion. It also pays to follow the links in APOD as they sometimes take you to strange places.

Posted in astro blog by Stuart on Thursday 10th Mar 2005 (14:53 UTC) | Permalink

Rosetta Flyby

The ESA's Rosetta spacecraft will be swinging by the Earth tonight as it makes its way to comet Churyumov-Gerasimenko. Closest approach will bring it as near as 1900 km to the surface, at 22:10 UT tonight and will then head off towards Mars to pick up more speed. It should reach about magnitude 8 or 9 so should be visible with a small telescope. Europe is supposed to be favorably placed for this event. Rosetta will move away from the constellation Sextans after sunset and move towards the direction of the sun, crossing the complete sky before it disappears on the horizon around 22 h UT.

Posted in astro blog by Stuart on Friday 04th Mar 2005 (18:39 UTC) | Permalink

Japanese SETI

Although the possibility that we are not alone in the universe is something people have thought about for thousands of years, the modern search for extraterrestial intelligence (SETI) started in 1959. It was then that Guiseppi Cocconi and Philip Morrison published an article in Nature about the possibility for alien civilizations to communicate using radio waves. SETI has continued to search for radio signals since then, so far without any success. Now, a team in Japan are going to give it a try using a 10m radio telescope and a 2m reflector telescope. Their survey will only last for 5 days so I don't hold out much hope of them finding anything unless they know something we don't!

Posted in astro blog by Stuart on Wednesday 02nd Mar 2005 (18:09 UTC) | Permalink
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