August’s evening skies are dominated by the Summer Triangle high in the south, made up of the three brightest stars in three different constellations, the stars Deneb in Cygnus, Altair in Aquila, and brilliant Vega in Lyra.
Vega is a magnitude 0 star, the fifth brightest star in the night sky, and the third brightest (after Sirius and Arcturus) visible to UK stargazers.
Look high in the south (almost directly overhead) in the late evenings in August and the very bright white star you’ll see is Vega. Look “down and left” of Vega and you’ll see the four other bright stars of Lyra in a rhombus shape.
Lyra represents the lyre of Orpheus, and it’s a great little constellation to observe through a telescope since, despite its diminutive size, is home to two Messier objects, M56 and M57.
Messier 56 (marked 2 above) is a loose globular cluster lying about 33000 light years away. It sits halfway along a line drawn between the “lowest” star of the lyre (the one furthest from Vega) and Albireo (beta Cygni). Through binoculars or a small telescope it looks just like a fuzzy star. You’ll need a pretty decent sized telescope (20cm+) to resolve individual stars.
Messier 57 (marked 1 above) is known as the Ring Nebula, and is one of the most-photographed of astronomical objects. It’s a planetary nebula, an expanding shell of gas that’s been puffed off by a giant red star in its death throes. Small scopes should show the elliptical shape but you’ll need a larger (20cm+) scope to see the hole in the middle and any features within the nebula. M57 is easy to find as it sits almost exactly half way between the two “lowest” stars of Lyra (the two furthest from Vega) beta and gamma Lyrae.
The bulk of this post was originally published on Prof. Andy Lawrence’s excellent blog, The e-Astronomer, which he wrote after reading some tweets I sent yesterday concerning the number of stars vs the number of raindrops that hit the UK in a year. I tweeted:
[1/4] Here’s a rain related #astronomy fact: Average rainfall in the UK last year (2012) was 1330mm or 1.33m Area of UK is 2.426×10^11 m^2
— Steve Owens (@darkskyman) July 28, 2013
[2/4] So approx. total amount of water that fell on the UK in 2012 is 3×10^11 m^3. The approx. average volume of a raindrop is 10^-9 m^3.
— Steve Owens (@darkskyman) July 28, 2013
[3/4] So the total number of raindrops that fell on the UK in 2012 is approx. 10^20 or 100,000,000,000,000,000,000 raindrops.
— Steve Owens (@darkskyman) July 28, 2013
[4/4] There are approx. 10^23 stars in the universe. So for every raindrop that hit the UK last year there are 1000 stars in the universe!
— Steve Owens (@darkskyman) July 28, 2013
Andy followed up:
Very nice, but I found myself thinking – how does the rate of raindrops compare to the rate of photons? The average raindrop flux is 13 raindrops/m^2/s. Lets compare to the Sun. The solar constant is 1360 W/m^2. If we take the typical photon as being at about 500nm with energy hc/λ = 4×10^-19J, we get roughly 3×10^21 photons/m^2/s.
So we get much much much more sunlight than rain! Woohee!
What about starlight? Well, as Mr Olbers pointed out, a Universe full of stars would make a sky as bright as the Sun in every direction. However, the Milky Way fades out, and the universe runs out too, in time and therefore space. So lets just get empirical. Cosmology types will often plot the ‘cosmic optical background’ at a level of about 10^-8 W/m^2/sr, about a factor of a thousand less than the CMB. However, that is the extragalactic background light; the summed emission from nearby stars is in fact much more. According to my Trusty Allen, star light from the whole sky is equivalent to 460 V=0 stars, or one star of V=-6.7. The apparent magnitude of the Sun is V=-26.7. So the scattered starlight is 20 magnitudes fainter or a factor of 10^8.
So in super-crude terms, starlight is giving us something like 3×10^13 photons/m^2/s. Still lots more than the 13 raindrops/m^2/s.
But what energy? Scaling down from the solar constant, starlight is giving us an energy flux of about 1.4×10^-5 W/m^2. What about raindrops? Each of those raindrops has mass 10^-6 kg. The terminal velocity of a raindrop depends on size, but at 1mm its about 4 m/s. So the KE per raindrop is about 8×10^-6 J and the energy flux is therefore 10^-5 W/m^2, about 6 times as much as starlight.
So… in particle count terms, the Sun wins hands down; starlight is down a factor of hundred million but still huge; and the raindrop count is pitiful, another factor of a trillion down.
In energy terms, the Sun still wins easily, with starlight a hundred millions times down; but the rain carries more energy than the starlight – just.
There are a variety of ways of measuring your night sky quality, and one of the most effective ways is by looking for the faintest star you can find with your naked eye, and noting its brightness, or magnitude. This provides what is known as Naked Eye Limiting Magnitude, NELM.
Of course just randomly casting about the sky for faint stars can lead you on a merry chase, and so a very useful method is to use one specific constellation – one you can always see, no matter what time of year – and look only at stars within that one constellation. This narrows the field somewhat, and makes your task that much easier.
For observers in Europe and North America the constellation of Ursa Minor, the Little Bear, provides an excellent choice for estimating NELM.
The overall shape of Ursa Minor is made up of seven bright-ish stars, but around and amongst these are many more fainter stars.
|Bright Star Name
|Ahfa al Farkadain (ζ)||4.25|
|Anwar al Farkadain (η)||4.95|
Even some of these “brighter” stars might not be visible from city centres. For example, if you are observing from a site with Bortle Class 8 you would not see η-UMi, while those unhappy stargazers under a Bortle Class 9 sky would only be able to pick out the three brightest stars, α-, β-, and γ-UMi. Only at Bortle Class 7 and darker will you make out all seven of the main stars of Ursa Minor.
But what if you’re at a good dark sky site? Well, you’re going to need a longer list of magnitudes, and a more detailed map of Ursa Minor.
|Star Number on
|Star Name||Visual Magnitude||Bortle Class
The stars in the map and table above have been numbered (by me – these aren’t official designations) from 1 to 19, with 1 (Polaris) being the brightest, and 19 (14 UMi) being the dimmest. You will only be able to see all 19 numbered stars from exceptionally dark places, virtually free of light pollution, what Bortle called “typical truly dark sky sites”. From my garden in the outskirts of a major city I can see numbers 11 and 12, but not number 13, giving me an NELM of 5.45.
Once again the Campaign to Protect Rural England and the British Astronomical Association’s Campaign for Dark Skies are running a UK-wide star count programme. This year’s event takes place between 20-27 January 2012. On any of these nights the skies will be dark enough to begin your star count by 7pm.
To make your own observations for Star Count 2012 find Orion in the sky and count how many stars you can see within the rectangular boundary formed by the four brightest stars in Orion. Those boundary stars are called Betelgeuse, Bellatrix, Rigel and Saiph.
You should count the three belt stars – Alnitak, Alnilam, and Mintaka – plus any other stars that are visible. The above star map shows around 40 stars within that boundary. If you can see that many stars then you’ll be in one of the darkest places in the UK. For most of us we’ll count far fewer stars than that. People in very bright urban areas may only see the three belt stars.
UPDATE: I should have mentioned that the CPRE will accept observations from anywhere in the UK, not just England.
Astronomers describe the darkness of the sky under which they are observing by referring to the limiting magnitude, that is the magnitude of the faintest star we can see.
We might use a very broad brush approach, and describe a sky as “fourth mag” (roughly what you’d see from the suburbs of a big city) or “sixth mag” (very dark indeed, with almost all but the very faintest stars visible).
The absolutely faintest stars anyone can see with the naked eye in perfect conditions (i.e. looking towards the zenith on a moonless night) are around 8th mag (this is for someone with exceptional vision), and so we refer to a perfect sky, utterly free of light pollution, as being magnitude 8.
However, most star maps and charts will only list stars down to a more modest limiting magnitude of 6.5, which number is often used as the standard limiting magnitude of a very dark sky.
The limiting magnitude of a sky has implications for how many stars you can see with the naked eye, and as a very rough rule of thumb for every one point of magnitude darker your sky is, you will see three or four times as many stars.
There are around 50 stars of magnitude 2 or brighter, which will therefore be visible in even the most light polluted skies with limiting mags of 2. Of course these 50 stars are spread over the whole sky so you’ll only ever see around half of them at once, and so in a very light polluted place like Manhattan, Las Vegas or Tokyo you can see around 25 stars only.
In the centre of other big cities, where the limiting magnitude is 3 you will see roughly 100 stars, and from the suburbs under magnitude 4 skies you will see around 300 stars (these star counts, and the ones that follow, are for half the sky only, i.e. in your visible hemisphere)
Once you get to a limiting magnitude of 5 in rural areas with a bit of light pollution you can expect to see 1000 stars or more. Further from big towns and cities, where your sky has a limiting magnitude of 6 you’ll see 3000 stars. At the limit of most star maps and charts, limiting mag 6.5, there are around 4500 stars to be seen in the sky at any one time, and so there are around 9000 stars altogether – over the whole sky – that are magnitude 6.5 or brighter. (As a little aside, the planetarium that I used to run in Glasgow Science Centre, had a Carl Zeiss starball projector that claimed to show 9000 stars, and so was obviously built to show every star that would feature on any standard star map or chart).
And beyond that you’ll need a telescope or binoculars to see more. Remember, those 9000 stars are only a tiny fraction of the 100,000,000,000 stars in our galaxy!
For the first time since this year’s GLOBE at Night started I have a clear sky, so I popped out into my back garden to find Orion and measure how light polluted my sky is.
I live in Glasgow, on the south of the city, in an area that is probably fairly described as being on the urban / suburban interface, so my sky isn’t great, even when clear. That coupled with regular cloud cover and tall trees to the south mean that my garden is far from ideal for stargazing. Even still, the small patch of sky visible to me never fails to impress on a clear night, even through the light pollution. At this time of year (late spring) Orion sits nicely in the space between the nearest buildings and the trees.
And tonight is no exception. Orion is standing proudly overhead, and using the GLOBE at Night star maps I could make out that my sky is magnitude four. Not bad, but not great. There are probably two or three times as many stars visible here as would be seen in the city centre of Glasgow, but go out into a truly dark site, such as Galloway Dark Sky Park, and you’ll see ten times as many again.
Sky Quality Metre
Using my nifty little Sky Quality Metre I can get a much more accurate measure of how bright my sky is. The metre gives a reading of magnitudes per square arcsecond (i.e. brightness per unit area in the sky). The readings, if taken of the zenith point directly overhead, range from around 16 in a bright city up to 23 in a very dark place indeed (Galloway Dark Sky Park, at the darkest part, registers 22.7 magnitudes per square arcsecond). The sky above my garden at the moment is reading 18.3, which is much better than I expected. I must get my telescope out before summer arrives…
Tonight, and for the next few nights, you can take part in the international project GLOBE at Night, an annual 2-week campaign. During GLOBE at Night, you will record the brightness of your night sky by matching its appearance against the constellation Orion with star maps of progressively fainter stars. Your measurements are submitted on-line and a few weeks later, organizers will release a map of light-pollution levels worldwide. Over the last four GLOBE at Night campaigns, volunteers from over 100 countries have contributed 35,000 measurements.
So what exactly do you need to do? It’s simple, just visit the GLOBE at Night website and follow their instructions. You’ll need to:
1. print out the Orion maps from the activity pack
2. go outside and find Orion. Make sure you wait till it’s properly dark, which is after about 2015 your time. Hint, Orion will be towards the south, and look like this:
3. compare what you see with the seven Orion maps, and note down which one it looks like the most. This will tell you the magnitude (brightness) of the dimmest stars you can see
4. report your observation here including the date and time you made the observation, and your latitude and longitude (don’t worry, there’s an interactive map that’ll help you find these)
Simple. Happy Orion spotting! I’m just waiting until it gets dark enough to make my own observations.