How many stars and what’s the limiting magnitude?

[Elixir]

Nothing that can be seen with the naked eye from earth’s surface is outside the Milky Way galaxy we live in.

The Magellanic Clouds and the Andromeda Galaxy are naked-eye objects.

Right. Shoulda said “every single star”.

 

[lpetrich]

Point of order: The nearest star is about 0.000004861 parsecs away, and at that distance the Sun would subtend an angle of about 0.0087 radians.

That's being pedantic. The Sun is a very exceptional case.

 Main sequence and  Red dwarf - one of the faintest stars is the red dwarf  LHS 2924 - visual luminosity 1.6*10^-6 times the Sun's and bolometric (total-flux) luminosity 2.2*10^-4 times the Sun's. its radius is 0.1 times the Sun's or about Jupiter's radius, and its surface temperature around 2000 K. Its mass is estimated at about 0.08 solar masses, and it is about 11 parsecs / 35 light years away. Its apparent visual magnitude is +19.35.

Less massive than that and it would not have nuclear fusion in its core, making it a brown dwarf.

 

[lpetrich]

Light Pollution and Astronomy: How Dark Are Your Night Skies? - Sky & Telescope

NELM = Naked-Eye Limiting Magnitude
To get what one can see with a 32-cm / 12.5-in telescope, add 9.0

Bortle scale	NELM
1: Excellent Dark-sky Site	7.6 to 8.0
2: Typical Truly Dark Site	7.1 to 7.5
3: Rural Sky	6.6 to 7.0
4: Rural/Suburban Transition	6.1 to 6.5
5: Suburban Sky	5.6 to 6.0
6: Bright Suburban Sky	5.5
7: Suburban/Urban Transition	5.0
8: City sky	4.5
9: Inner-city Sky	4.0

 

[lpetrich]

That table looks like a table of best-case visibility.

Limiting Magnitude | Astronomics.com
For observing with a telescope: 7.5 + 5 log10 aperture (in cm)
For 32 cm, the limiting magnitude is 15.0, and using the above table's scaling value, the limiting magnitude without a telescope is 6.0.

 Limiting magnitude

There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.

From relatively dark suburban areas, the limiting magnitude is frequently closer to 5 or somewhat fainter, but from very remote and clear sites, some amateur astronomers can see nearly as faint as 8th magnitude. Many basic observing references quote a limiting magnitude of 6, as this is the approximate limit of star maps which date from before the invention of the telescope. Ability in this area, which requires the use of averted vision, varies substantially from observer to observer, with both youth and experience being beneficial.

 

[crazyfingers]

Scene Viewer

Create 3D scenes in your browser and share them with the world.

www.arcgis.com

ETA strange title but it goes to a google map type representation of light pollution.

Here is an interactive globe of light pollution

Here is the story about it.

Light Pollution Is Creeping Up on the World's Observatories

Astronomical observatories enjoy some of the world’s darkest night skies. But even there light pollution is spreading, a new study suggests.

skyandtelescope.org

 

[bilby]

First Light Optics have a free app available called 'Clear Outside', which predicts astronomical viewing conditions at a given location based on weather, moon phase, and local light pollution levels.

 

[SLD]

That table looks like a table of best-case visibility.

Limiting Magnitude | Astronomics.com
For observing with a telescope: 7.5 + 5 log10 aperture (in cm)
For 32 cm, the limiting magnitude is 15.0, and using the above table's scaling value, the limiting magnitude without a telescope is 6.0.

 Limiting magnitude

There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.

From relatively dark suburban areas, the limiting magnitude is frequently closer to 5 or somewhat fainter, but from very remote and clear sites, some amateur astronomers can see nearly as faint as 8th magnitude. Many basic observing references quote a limiting magnitude of 6, as this is the approximate limit of star maps which date from before the invention of the telescope. Ability in this area, which requires the use of averted vision, varies substantially from observer to observer, with both youth and experience being beneficial.

But the limiting magnitude of a camera is far more than the naked eye. I look through the very scope that took those pics and there is no way I see that many. Very few in fact. But let the shutter open for 2 hours total and beaucoup stars show up.

 

[Shadowy Man]

That table looks like a table of best-case visibility.

Limiting Magnitude | Astronomics.com
For observing with a telescope: 7.5 + 5 log10 aperture (in cm)
For 32 cm, the limiting magnitude is 15.0, and using the above table's scaling value, the limiting magnitude without a telescope is 6.0.

 Limiting magnitude

There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.

From relatively dark suburban areas, the limiting magnitude is frequently closer to 5 or somewhat fainter, but from very remote and clear sites, some amateur astronomers can see nearly as faint as 8th magnitude. Many basic observing references quote a limiting magnitude of 6, as this is the approximate limit of star maps which date from before the invention of the telescope. Ability in this area, which requires the use of averted vision, varies substantially from observer to observer, with both youth and experience being beneficial.

But the limiting magnitude of a camera is far more than the naked eye. I look through the very scope that took those pics and there is no way I see that many. Very few in fact. But let the shutter open for 2 hours total and beaucoup stars show up.

Yes. You can think of the light coming from stars as photons per second per square meter. So, the amount of light you collect in an image is proportional to both the length of the exposure and the size of the aperture of your telescope. These are both small for the human eye but substantially larger for even the most modest of cameras. And with better resolution one can concentrate that light into a smaller area, thus achieving greater contrast against the sky for even better visibility.

 

[SLD]

That table looks like a table of best-case visibility.

Limiting Magnitude | Astronomics.com
For observing with a telescope: 7.5 + 5 log10 aperture (in cm)
For 32 cm, the limiting magnitude is 15.0, and using the above table's scaling value, the limiting magnitude without a telescope is 6.0.

 Limiting magnitude

There is even variation within metropolitan areas. For those who live in the immediate suburbs of New York City, the limiting magnitude might be 4.0. This corresponds to roughly 250 visible stars, or one-tenth the number that can be perceived under perfectly dark skies. From the New York City boroughs outside Manhattan (Brooklyn, Queens, Staten Island and the Bronx), the limiting magnitude might be 3.0, suggesting that at best, only about 50 stars might be seen at any one time. From brightly lit Midtown Manhattan, the limiting magnitude is possibly 2.0, meaning that from the heart of New York City only approximately 15 stars will be visible at any given time.

From relatively dark suburban areas, the limiting magnitude is frequently closer to 5 or somewhat fainter, but from very remote and clear sites, some amateur astronomers can see nearly as faint as 8th magnitude. Many basic observing references quote a limiting magnitude of 6, as this is the approximate limit of star maps which date from before the invention of the telescope. Ability in this area, which requires the use of averted vision, varies substantially from observer to observer, with both youth and experience being beneficial.

But the limiting magnitude of a camera is far more than the naked eye. I look through the very scope that took those pics and there is no way I see that many. Very few in fact. But let the shutter open for 2 hours total and beaucoup stars show up.

Yes. You can think of the light coming from stars as photons per second per square meter. So, the amount of light you collect in an image is proportional to both the length of the exposure and the size of the aperture of your telescope. These are both small for the human eye but substantially larger for even the most modest of cameras. And with better resolution one can concentrate that light into a smaller area, thus achieving greater contrast against the sky for even better visibility.

So what’s the magnitude of the star that’s circled, likely to be?

And am I correct that it’s probably an M class dwarf?

 

[Shadowy Man]

So what’s the magnitude of the star that’s circled, likely to be?

And am I correct that it’s probably an M class dwarf?
View attachment 42822

There’s not enough information to answer those questions. Where in the sky is this? What hardware was used to take this photo? Were filters involved? What is the exposure time of the image? What is the stretch in the image?

 

[SLD]

So what’s the magnitude of the star that’s circled, likely to be?

And am I correct that it’s probably an M class dwarf?
View attachment 42822

There’s not enough information to answer those questions. Where in the sky is this? What hardware was used to take this photo? Were filters involved? What is the exposure time of the image? What is the stretch in the image?

In Cygnus, just south of Sadr. No filters, just a color camera with 3.96um pixel size, 71mm aperture and 336 focal length, 2 hours of 3 min subs.

of course it’s significantly cropped. Original is in the Astrophotography thread

 

[Shadowy Man]

So what’s the magnitude of the star that’s circled, likely to be?

And am I correct that it’s probably an M class dwarf?
View attachment 42822

There’s not enough information to answer those questions. Where in the sky is this? What hardware was used to take this photo? Were filters involved? What is the exposure time of the image? What is the stretch in the image?

In Cygnus, just south of Sadr. No filters, just a color camera with 3.96um pixel size, 71mm aperture and 336 focal length, 2 hours of 3 min subs.

of course it’s significantly cropped. Original is in the Astrophotography thread

Yeah, so that’s the start of the info you would need to get at a magnitude of that star.

The probability may favor it being an M star simply because lower mass stars are more common, but to know the spectral type of the star requires at least obtaining a “color” (from the difference of brightness in certain bandpass filters and accounting for possible interstellar reddening) and/or a spectrum.

 

[SLD]

I looked at the area in stellarium and saw that it had stars of magnitude 19. But these were clearly brighter than this because they were spaced apart. There were far less than I see here. I wonder if we are looking at a mag 26 or dimmer star.

 

[Bomb#20]

So what’s the magnitude of the star that’s circled, likely to be?

And am I correct that it’s probably an M class dwarf?
View attachment 42822

There’s not enough information to answer those questions. Where in the sky is this? What hardware was used to take this photo? Were filters involved? What is the exposure time of the image? What is the stretch in the image?

In Cygnus, just south of Sadr. No filters, just a color camera with 3.96um pixel size, 71mm aperture and 336 focal length, 2 hours of 3 min subs.

of course it’s significantly cropped. Original is in the Astrophotography thread

If there are no filters then it's not a red dwarf, with probability about 99.9%. You can tell, because there are a few thousand naked-eye stars in the sky, and three of them are M class. Red dwarfs are the most common type, but they're so dim that among all stars at a given magnitude they're very rare. That principle applies to telescopes exactly as it applies to the naked eye.

 

[Shadowy Man]

Recently I was talking with some colleagues about using Betelgeuse as a calibration star for an infrared instrument I have been working on and they asked if we could use any brighter stars. I said that Betelgeuse is the brightest star in the sky in the infrared. It is an M class supergiant with an apparent magnitude of 0.5 in the visible but -4 at 2 microns. That’s over 60 times brighter at 2 microns!

 

[barbos]

Well, I mean, look at the picture. It’s so full of stars that almost every point is covered by the surface of some Star.

That picture was taken by a telescope (?) and god knows how long it took to collect all the light.