In the live video chat over the weekend, I was asked a GREAT question: can you see more stars from a high altitude, like say from an airplane? This is a common question, and it makes the assumption that the Earth’s atmosphere absorbs starlight; so if you go up high enough you’ll see lots more stars.
Turns out, not so much. Here’s why:
If you’re curious, the image I put in the video from Hubble is reproduced below, but you really need to go take a look at the high-res version. It’s not color, but it’s still very cool.
Also, the beautiful image of Orion is from J P Stanley’s Flickr set (reproduced under Creative Commons licensing, so if you like his picture go show him some love on Flickr).
July 20th, 2008 at 11:36 pm
But what about light pollution?
July 20th, 2008 at 11:51 pm
Also because the stars look “crisper” — i.e., there is no seeing – telescopes are able to image fainter stars since the light is not as spread out over a broad distribution. This wouldn’t apply to the human eye, whose diffraction limit isn’t that precise.
Another slight improvement — you would be able to see stars in any part of the sky equally clearly. You would not get the secant zenith airmass effect, where stars near the horizon are obscured by looking through a long length of sky.
July 20th, 2008 at 11:52 pm
On the other hand, at high altitudes, there isn’t likely to be a sprawling metropolis to ruin the view.
The best sky I ever saw was on top of a 10,000 foot mountain, and the altitude DID have the positive effect of ensuring that no city would ever be built nearby to give off light pollution.
July 20th, 2008 at 11:52 pm
I, too, thought about standing on the mountain and the lack of light pollution….but Phil mentioned more anthro-based problems so perhaps he thought pollution, etc covered light also.
But, as an aside, I linked to other BA youtube posts….and came across the Kill-O-metRE one. Phil, I gotta say man….I wager my salary for you to find a road sign that says “KpH” anywhere in my homeland of CANada
Km/h and not KpH
cm/s and not cmps
m/s and not mps
…
A little tidbit to clear my mind. Tidbit, not timbit.
T
July 20th, 2008 at 11:58 pm
I was a bit of a fool and posted this question at YouTube rather than here. This is a direct copy of the comment at YouTube:
————-
was always under the impression that the MFP of photons in the atmosphere was, well, not its thickness.
The extinction coefficient Q is a function of effective cross-sections, and the change in apparent magnitude is approximately the number of optical depths along the line of sight, which is approximately Q*(column density). This number shouldn’t be *huge*, but should it really be that negligible?
Carroll & Ostlie, “Introduction to Modern Astrophysics”, 1st ed. pp266-268;439-441
————-
Best as I can figure, atmospheric dust is 0.5-2 µm or so … this is right around the region where extinction should be important at optical wavelengths. Then again, as I mentioned, I am not sure how many optical depths the atmosphere is at 200-700 nm, even across the first scale height.
Alas. As usual, I’m nice and heavy on the theory but a bit light on the whole “practical” thing. Well, if the light is shorter than 21 cm or thereabouts ….
July 21st, 2008 at 2:01 am
Only 3000 visible stars? Wow. I thought it would be a lot more (provided you’re away from any major light pollution).
July 21st, 2008 at 3:53 am
That pilot, flying over a major city, will definitely see many more stars (in principle – not sure how good a view the cockpit windows will provide), simply by being above the bulk of the glowing atmosphere.
Light pollution is definitely the biggest factor that Phil didn’t account for.
July 21st, 2008 at 5:08 am
I bet I could see a lot more from a high altitude. The light pollution in central London is awful.
July 21st, 2008 at 5:53 am
Several astronauts – with astronomical background or at least ‘amateur’ interest – have noted in recent interviews that e.g. the Milky Way looked crisper from the ISS (under right conditions) than they had ever seen from Earth. This may be purely subjective thoughts (no limiting magnitude measurements or star counts), but they say so.
July 21st, 2008 at 6:01 am
Here’s a further refinement of the question. Okay, so atmospheric light-absorption is a non-issue, but what about the horizon? At 30,000 ft wouldn’t the amount of the sky blocked by the Earth be reduced compared to the ground as the visible horizon is more distant?
July 21st, 2008 at 6:44 am
Sorry, Phil … your explanation seemed contradictory to me.
Your broad answer to the question is NO; you won’t see more stars from above the atmosphere. But you go on to explain that you would be able to see fainter objects from above the atmosphere. As when moving away from light pollution, that equates in my experience to the ability to see a greater number of objects as the limiting magnitude changes. Seems to me that the answer then is YES, if only by a small number.
July 21st, 2008 at 6:45 am
Phil, the absorption of light from a star at the horizon is huge compared to the absorption of starlight at the zenith, because the path through the atmosphere is so much longer. In that sense it helps a lot when you’re at 11km altitude. In fact, this is why people are now pushing for single-photon quantum cryptography via satellites to cover large distances, rather than through line-of-sight communication.
July 21st, 2008 at 7:26 am
The original question was not “how many more stars can I see at high alitude (or in space) than I can from a large urban center” – it was how many more stars can I see than from the surface of the earth. Phil is interpreting the question as “How many stars does the atmosphere filter out, that I would otherwise see”. So, compare with say the Australian outback. I also presume he is using mag 6 as the cutoff of human perception (which is a nice round number, though many can see deeper, and many not that deep). Removing the extinction (2%) and tightening the stars to finer points (increasing apparent surface brightness) would effectively brighten all stars by, what, 0.2 magnitudes? So that would bring stars of 6.2 mag (from the surface) into the “visible” category.
Though even 1000 more stars, or 2000 more, were technically visible, they would be just barely visible – just on the limit of perception, it is still the bright stars you see and can count that give you the impresion of how many stars there are.
July 21st, 2008 at 7:30 am
Well, you shouldn’t forget all the stars that are blocked by that big ugly planet you’re standing on.. that’s half the stars in the sky!
July 21st, 2008 at 8:31 am
Ugly? Haven’t you seen the pictures of it from Mars?
It’s a lovely blue.
From a distance…
July 21st, 2008 at 8:43 am
I agree with you about the light pollution. I live in a relatively small town near Vancouver, but far enough away not to be affected by it’s “dome of light”. I took my telescope out to what I thought would be a great location, only to be very disappointed by the fact that my viewing conditions were atrocious due to light pollution. I think our town council just passed a resolution to use less light polluting fixtures on street lighting. That does not solve the primary problem of those huge orange sodium (?) lights. I’ve read many accounts of kids going out of big cities to the country side and looking up to the sky at night and being utterly amazed because of what they can see.
July 21st, 2008 at 8:50 am
I don’t mention light pollution specifically, but I do say that the sky itself glows which is the same thing. That’s a big factor, of course, and that’s one of the three reasons I gave for putting telescopes like Hubble in orbit.
My point is that sure, you can see some more stars — very faint ones — but in general you don’t see that many more. It’s not like you’re seeing zillions more.
July 21st, 2008 at 9:03 am
Though I take the point regarding “not seeing zillions more” – your sight at the surface as has been noted is limited by the horizon. As you increase height, your horizon increases – until you get to the ultimate – space and beyond Earth. This in effect increases your horizon.
If we retain the number of 3000 visible stars at the surface, as your altitude above the surface increases, your scope of visible stars also increases. 300 more stars is a 10% increase.
Since this is just after the anniversary of the first moon landing, I submit that Neil and Buzz, had they not been in full sunlight the entire time, would have experienced nearly (thought not quite) a 100% increase in visible stars – to perhaps nearly 6000. Certainly during the transit at least.
This assumes an even distribution of stars over Mag-6 and ignores that while on the moon itself, they have a similar (though closer) horizon.
So – I would say that at the most, while in space and no where near the Earth, the most you could see was something in the order of 6000 stars.
JC
July 21st, 2008 at 9:12 am
It is not just, and I think not even primarily, atmospheric extinction to consider. I missed one element that I think is non-trivial: the light scattering characteristics of our atmosphere. The very reason there is a twilight when the sun is below the horizon, instead of darkness-at-once.
Here at 51 degrees latitude I am very familiar to the effect that it does notably *not* get entirely dark from mid-May to mid-July: because the sun never is more than 15 degrees below the horizon. The atmosphere is scattering sunlight even when teh sun is notably below the horizon. This is enough to produce a visible eternal twilight, certainly when looking towards the northern horizon. In winter this effect is not notable, but I am sure that to some degree the sky background will still be brighter, drowning faint objects, than without an atmosphere.
You can also see it at altitude: the (daylight) sky seen from the top of a Himalaya peak looks darker than when seen from sea level. That’s the same liht-scattering effect of the atmosphere at work.
July 21st, 2008 at 9:15 am
Oh: and for the same reason the moon is still visible when eclipsed, again pointing out the magnitude of the light scattering capabilities of our atmosphere.
And sorry for the typoes in my contribution above: crappy keyboard…
July 21st, 2008 at 9:57 am
it’s probably true that under ideal atmospheric conditions you’ll see the same naked eye stuff from Earth or space, but when you’re telescoping for stuff dimmer than 20th magnitude, you start running into the fact that the atmosphere itself glows at about 21-22nd mag. So while you won’t see a difference, your telescope will.
July 21st, 2008 at 10:25 am
Just how big things can can we put in orbit?
The shuttle imposes certain limitations on dimensions, of course, but since that’s also an enormous dead weight, let’s think about automated missions.
So … ten metres is prolly too heavy (even with those awesome ‘hollow core, honeycomb’ mirrors they’re casting under a sports stadium somewhere, but what about three or four or even five? How heavy and wide is too heavy and wide?
July 21st, 2008 at 10:42 am
Sili-
If you send up Keck-style segmented mirrors and assemble them in space, there’s no limit to the size of telescope we could build in space. But it’s cheaper to build them on the ground and use adaptive optics to get around atmospheric effects.
July 21st, 2008 at 11:00 am
Howdy-
This video inspired me to consider shopping around for a casual telescope. Since I enjoy watching you discuss astronomy -and- I would also like your insight, I was hopeful you could do a segment on telescopes, the different kinds, their ranges, and what kind of sights we can expect from them?
Or is this too much?
Regardless, watching this video has inspired me to take my love for space and translate it into something more.. personal.
July 21st, 2008 at 11:15 am
Again, thanks.
July 21st, 2008 at 11:30 am
Thank you, mr Beaton,
The segmented mirrors are fancy too, but I was talking about Steward Observatory Mirror Lab (site unfortunately down at the moment).
I just thought that since Hubble is such a success, Hubble++ would be an even bigger boon (not least if they get the grinding done right this time).
July 21st, 2008 at 11:38 am
Bobby – click on BudgetAstronomer above, and check out the FAQ section. There are also pages devoted to types of telescopes and recommendations.
July 21st, 2008 at 11:43 am
Several months ago, I took a night flight from Florida to New York. (Not only less atmosphere at 30,000 feet, but being over the Atlantic meant virtually zero light pollution.) Looking out the cabin window certainly _seemed_ to have a much better view of the sky. While I didn’t count the number of stars, everything was certainly “crisper”. And it certainly appeared to me that I was able to make out much more of Orion and M42 than usual.
This is, of course, purely subjective.
July 21st, 2008 at 12:46 pm
[...] Phil Plait at Bad Astronomy discusses whether or not you can see more stars from space. [...]
July 21st, 2008 at 3:17 pm
I’ve often wondered that very thing. Thanks for explaining it so well Phil! Now I’m extra mad at myself for missing the chat!
July 22nd, 2008 at 3:16 am
I think you will see more stars at 30,000 ft.
First, extinction in the visual is 10%, not 2% as TBA said.
Second, this is looking straight up. Looking at 60 degrees to the zenith
(where most of the stars are), it increases to 20%. And more lower down.
[The fact that light pollution will probably be less on, say, a midocean part of an
intercontinental flight that almost anywhere on the Earth's surface will also help,
but I'm not sure by how much.]
So my guess, is that you can see an extra 600 or so stars.
Two factors to think about.
First, this assumes X% more light = X% more stars, but the actual increase
depends on the distribution of the number of stars per magnitude at 5th magnitude.
Second, can you only see 3000 stars? I think this is the number down to 5th, but the
fully adapted eye can see much deeper.
July 22nd, 2008 at 7:43 am
I love learning new thing, especially when they contradict some of those old things I’ve had stored in my brain in the past. Keep it up Phil!
July 22nd, 2008 at 9:46 am
Diego aid:
> Here’s a further refinement of the question. Okay, so atmospheric light-absorption is a non-issue, but what about the horizon? At 30,000 ft wouldn’t the amount of the sky blocked by the Earth be reduced compared to the ground as the visible horizon is more distant?
Yes, you would see a bit more sky than ground. But that is irrelevant to the density of stars you would see, which is the actual question.
JackC said:
> Though I take the point regarding “not seeing zillions more” – your sight at the surface as has been noted is limited by the horizon. As you increase height, your horizon increases – until you get to the ultimate – space and beyond Earth. This in effect increases your horizon.
True.
> If we retain the number of 3000 visible stars at the surface, as your altitude above the surface increases, your scope of visible stars also increases. 300 more stars is a 10% increase.
False. You are misunderstanding the original number. 3000 stars means 3000 stars in the whole sky, 360 degrees around Earth. Your observable number is smaller because on the surface you cannot see the whole sky at one time.
The question is not how much sky can you see. It is about whether the sky you see has more dots in it, or brighter dots. That is an image density question, not an area question. Sure, at 30,000 ft you see a bit more area, but the density is not dramatically increased. In a space suit floating in space, you might be about to turn around and view the entire sky (except maybe some tiny fragment blocked by your pesky space vehicle), but it won’t have any where the dramatic difference of experience as going from an urban street to a remote field (like in the Austrailian outback).
July 22nd, 2008 at 12:21 pm
Irishman: “False: you are misunderstanding the original number.”
I doubt I am, Irishman. I seem to have lost an ability to cut-n-paste here, but at the very least, Google “how many stars can be seen” (or words to that effect) and you will come up with many good links stating something akin to “look up into the sky on a clear night, away from strong lights, and you will see a few thousand individual stars.” (roughly taken from an ESA site). I take “A few thousand” to be the 3,000 number.
On Earth, your horizon is roughly 8 miles away at sea level (very roughly). You do not, even in the best of situations, have even half of the celestial sphere above you. In that “limited” region of sky, it is generally assumed (again, very generally) that about 3k stars can be seen with the average naked eye.
As your altitude increases, your horizon increases and the number of visible stars increases as a direct result – just not as much as many people would think. Certainly not in the thousands until you have a completely unobscured view (which DOES give you a 360-degree view) – which does not happen unless you remove the horizon completely. Of course, this is generally theoretical and pretty much ignores the atmospheric effects well noted above.
I have never seen referenced ANYWHERE that the 3k stars reference number is for a full 360-degree ignore-the-Earth-is-there scenario. If you have a cite for that, and can paste it (unlike me!) I would love to see it.
You make the same point, I believe, in your last paragraph. Correct – the density does not (significantly, with suitably large values for significant) increase, but your horizon does – and assuming an equal distribution of unaided visibility, you therefore get more visible stars.
Just not more than 100% more – again ignoring all the mostly minimal atmospheric stuff. All other points well taken.
JC
July 22nd, 2008 at 4:50 pm
Considering your curious ability to spot Plankton in a hot dog bun, I’m surprised you didn’t notice the playboy bunny right above Orion.
July 22nd, 2008 at 5:09 pm
You might enjoy this article from S&T… http://www.skyandtelescope.com/howto/visualobserving/19712459.html
From sea level, ignoring the aersol optical depth factor (AOD), the brightness of stars is reduced to about 86% due to scattering alone. Using the normal AOD factor range (0.2 to 0.5 per air mass), the brightness range is now about 50% to 70% of that seen from space.
From Mauna Kea (4200 meters), with no AOD, they see about 92% of AM0 (space view) when looking straight up, and about 90% at a 60 deg. alt.
The stellar magnitude loss from sea level is more than 0.6. Thus, from space, the number of stars will almost double the sea level viewers, and this is just in the view overhead. At 60 degress from zenith, four times as many stars should be seen from space in this region.
August 1st, 2008 at 2:45 pm
Very cool that the BAD ASTRONOMER himself answered one of my questions!! There’s been a lot of flak on his blog over him not giving the “right” answer. The truth is that I really didn’t ask the “right” question. I should have asked “Hey Dr Plait, I notice that when I am flying at 35k feet I see a lot more stars. How much of this is due to the lack of light pollution and how much is due to decreased atmospheric interference?” Instead I asked “How many more stars can I see since I’m above most of the atmosphere?”
To the people who pointed out the reason I see so many stars, thanks. Your responses are in the spirit of good, rational discussion the likes of which the scientific method greatly benefits from. But to those of you who naysayed the BA, I say “No way!”
My poorly phrased question wasn’t Phil’s fault, it was my own. He took my rambling, poorly thought out question and turned it into a learning opportunity for us all. Thanks to you Dr. P!