# Tag Info

41

Naked eye stars are not distributed uniformly in the sky. That is because the median naked eye star is at a distance of 440 light years, and this is far enough away that some of the details of Galactic structure start to become apparent. Most importantly, the density of stars increase towards the Galactic midplane and has a scale height of a few hundred ...

27

Olber's Paradox was created at a time before the idea of a finite universe was accepted. (It was thought of in the 1600's). In order to resolve Olber's Paradox, you have to introduce the idea that either the universe had a beginning or it is of finite size. (Note: the solution does not require an expanding universe). So, at the time, it was a paradox. ...

27

I'll turn my comment into a full answer. To put it simply, we actually do see the Milky Way all around us, even in the diametrically opposite direction from the galactic core. You can see this in the image below which is a full sky image I took from APOD. If you look at the edges of the disk in that image, you're looking at what is actually the edge of our ...

22

It varies. The best astronomical sites have a visual band extinction of 0.1 mag, which means that only $\sim 10$ per cent of light is absorbed/scattered in the atmosphere. In dusty, smoggy or polluted sites, this can easily reach one magnitude of extinction, which means that 60 per cent of the light is scattered. These numbers are per airmass - which ...

16

Astrometry.net has identified your star field as being part of the Andromeda constellation. The diffuse object in the centre of your image is the Andromeda Galaxy (Messier 31). The bright star to the left of it is Mirach (β Andromedae). Given a reasonably dark sky and averted vision, it is possible to see the core of M31 with the naked eye. Since M31 is ...

13

I think your requirements can be met by Stellarium. It is a freely available open-source planetarium software available for PC, and can be used offline. There is also a web version, which you can try out here. You can filter stars out by pollution levels, as illustrated in the stellarium wiki. Here is an Astronomy Stackexchange answer on matching the ...

12

There is an accepted answer already, but there is a couple known cases of a star we know has gone supernova, and yet we can still see it. This source describes one such unique circumstance. The star that exploded happened to be in a galaxy that was behind another massive one from our point of view. The alignment was just right such that the light formed ...

12

The Earth is a sphere (or is nearly a sphere). So to make a map of the whole Earth you would need to project it onto a flat surface. When you do this you create distortions. For example, on many maps, the straight line between New York and Japan looks like a long curve. It is not actually a curve, but when you stretch the surface of Earth to make it flat, ...

12

Satellites just add moving lights to the sky, they do not obscure stars. However, some may find that disrupting their view of what a sky should look like. The visual magnitude of starlink satellites is about 5.92 at zenith, and usually darker (but also sometimes brighter). The new darker satellites are about 0.77 magnitudes fainter. This is in the lower ...

11

To answer this, I interrogated the revised Hipparcos catalogue produced by van Leeuwen (2007), accessed at the CDS, Strasbourg. There are 4559 stars in the catalogue with a Hipparcos magnitude $<6$ and which you might class as "naked eye" objects, though you'd need good eyes, a good site and plenty of patience to be dealing with stars fainter than about ...

10

Before we begin: how far a planet is seen from the Sun is called elongation, and it's measured in degrees. 0° elongation means it's right on top of the Sun (or behind); 180° elongation means it's opposite to the Sun (it's highest in the sky at midnight, when the Sun is on the other side of the Earth). https://en.wikipedia.org/wiki/Elongation_(astronomy) ...

10

It's most probably Sirius. At this time of year (at 1 am local time) it's low in the sky in the East, so there is a lot of atmosphere in the way, and as Sirius is a bright bluish star, it will show all the colours described as it twinkles.

10

The moon is quite interesting as you get near the poles. It still moves round due to the Earth's rotation once a day, but it also orbits with a period of a month so what you get is two weeks of moon above the horizon followed by two weeks where you cannot see it. What changes through the year is what phase you see when the moon is visible, and in the middle ...

8

It's a pretty straight forward calculation of 3 factors. Distance from the sun, apparent size and albedo. I'm going to compare Jupiter to our full moon, since we're all familiar with that. Jupiter averages between 4.95 and 5.45 AU from the sun. That puts it, in comparison with our full moon (about 1.02 AU on average), that means individual square ...

8

The question has been addressed, but for completeness I should like to remark that the most thorough and readily understandable discussion of Olber's Paradox is that of E. Harrison in his book Cosmology, the Science of the Universe (CUP 2000). It is also worth remarking that we now know that the sky at night is not in fact dark: the light from the cosmic ...

8

Most high-redshift galaxies — i.e. the very distant ones, billions of light years away — are detected due to the UV light that comes primarily from very hot stars (in particular, copious amounts of Lyman $\alpha$ photons at 121.6 nm are emitted from the gas surrounding the hot stars which is ionized and then recombines). Because these stars "burn their fuel" ...

8

I have tried to collate some info from various other answers. Density of stars where we are: About 0.15 per cubic parsec Density of stars "at the middle of the galaxy". Unfortunately I don't know how "100 ly" affects this. It's probably about this figure. About 50-100 stars per cubic parsec So in this QA it's "about 500x as dense&...

8

Alright I finally finished this program so I could take a look at each tier individually and see for myself. First of all, the projection type does indeed matter, so I will explain it here. It needs to be an equal-area projection. The whole point of the question was about a uniform distribution of stars over the surface of a sphere. In other words, each ...

7

The main effect would be that the Milky Way would become much more prominent and asymmetric. At the moment, our view into the Galactic plane is limited to around 1000-3000 parsecs by dust. If you look at the Galactic latitude distribution of naked eye (Aren't there more naked-eye-visible stars in the Milky Way plane? ) you see that most naked eye stars ...

7

You are, I think, asking two very similar but linked questions: Are there stars - in our galaxy, as stars in other galaxies are not individually visible - that are 'dead' but still alight to us. The Slate article you linked is rather good in this respect: i) the lifespan of a given star is much longer than the time taken for light to reach us so it is ...

7

As you have already identified the objects you were seeing, I'll explain the effect you were seeing. In that situation there are three things to consider: atmospheric chromatic dispersion, seeing and human color perception. Light entering the atmosphere is refracted, because of the changing speed of light in air compared to the vacuum of space. The amount ...

7

At any one time, only half the sky can be visible, and half is below the horizon. At the poles the sky appears to rotate about the zenith, and so no new parts of the sky ever become visible. At the North pole, you can never see any stars in the Southern hemisphere. On the equator different parts of the sky become visible and over the year all the stars ...

6

The Moon, Mars, Venus are seen against a dark sky using an eye whose aperture is adjusted to the average brightness of the visual field. So bright small sources are over-exposed and so appear bright. Photographs of planetary surfaces are taken with aperture and exposure set for proper exposure. So appear closer to how they would look if they filled your eye'...

6

As most people are aware when they play with bar magnets, magnetic poles repel magnets of the same polarity and attract opposites. Magnets have two polarities: north and south. Electric charges are classified as positive and negative, and also attract opposites and repel charges of the same sign. However, magnetic poles and electric charges do not match ...

6

There's a recent study on this, based on satellite and ground observations around the world. According to the paper: The Milky Way is hidden from more than one-third of humanity, including 60% of Europeans and nearly 80% of North Americans. And there's a map: The New World Atlas of Artificial Sky Brightness. This is probably the most accurate and up to ...

6

Googling for 'light pollution map' gives a pretty good looking result. Those things usually are created using satellite measurments that are susceptible to scattered light (by measuring the polarity of light) in order to separate direct illumination from actual light pollution. I also suggest you visit a city and then compare the light pollution index on ...

6

We can see Venus at night (dusk) and in the morning (predawn), just not all night or every night. We need some geometry to understand this, and it boils down to tangents. I've shamelessly borrowed this diagram from a paper to show the idea. It's discussing Mercury, but it's the same idea for Venus. We'll simplify things by using the pretty good ...

6

Taking Mars' average distance from the sun of 1.52 AU, the sun would be 43% as bright. Phobos has an albedo of about 0.071 which is pretty dark. Darker than the Moon with an average albedo of about 0.12, so it reflects about 59% as well as the Moon does. That gives it a brightness to area of just over 25% the brightness of the moon. Phobos is an odd ...

6

Sirius is the brightest star in the sky (excluding the Sun, of course), with a visual apparent magnitude of −1.46, and is visible to everyone in both Argentina and Italy, depending on the season. In fact, Sirius is visible from anywhere in the Southern Hemisphere, including all of Argentina (which, I might add, is an enormous country stretching from just ...

6

The logic that In-The-Sky.org uses to decide whether events are observable or not doesn't currently work very well for Mercury and Venus. I have some plans to fix this in the long term, but always have far too many projects on the go. As you correctly say, it's blazingly obvious that Mercury is highest in the sky at lunchtime, and the code I wrote to ...

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