I Is the Sun considered a dwarf star?

[Roach DN]

Is our Earth's sun is a dwarf star?

 

[phyzguy]

In some references you will see anything that isn't a giant or supergiant star referred to as a dwarf star, but it would be more accurate to refer to our sun as a main sequence star.

 

[kimbyd]

Is our Earth's sun is a dwarf star?

If you're curious as to why main sequence stars are sometimes called dwarf stars, this video that compares the sizes of a number of stars might be instructive:


Note that the vast majority of stars in our galaxy are main sequence stars, and close in size to our own Sun. The giant stars shown in the above video are less common, at least within our galaxy for the time being. Stars that are more than about 1/4th the mass of our Sun eventually pass through a giant phase. When our Sun enters its red giant phase in about 5 billion years, the surface of the Sun will extend past where the Earth's orbit currently is (the Earth itself will increase its orbital distance due to the Sun losing mass as it goes into the giant phase, so it may not be swallowed).

 

[Drakkith]

In some references you will see anything that isn't a giant or supergiant star referred to as a dwarf star, but it would be more accurate to refer to our sun as a main sequence star.

Indeed. I believe that all stars larger than "dwarf" stars have passed beyond the main sequence phase, correct?

 

[mathman]

I think you are quibbling about a definition. Usually dwarf stars refer to stars much smaller than the sun. red dwarfs - small main sequence; white dwarfs - end stage (smaller than neutron); brown dwarfs - stars emitting radiation but too small for fusion

 

[rootone]

Sun is a main sequence star, a bit bigger than average, but nothing special.
The Sun is mostly interesting because it's a solo star, and most star systems are binary, or moreso.

 

[Vanadium 50]

Sun is a main sequence star, a bit bigger than average, but nothing special.
The Sun is mostly interesting because it's a solo star, and most star systems are binary, or moreso.

The only thing that's true is that the sun is a main sequence star. It is ~3x heavier than the average, it is bigger than ~90% of the stars out there, and the (large) majority of star systems are singles.

 

[Ophiolite]

the (large) majority of star systems are singles.

This seems to be contentious. For example
Chandra X-Ray Observatory
"More than 80% of all stars are members of multiple star systems containing two or more stars"

Atlas of the Universe
"About half of all stars are found in systems containing two or more stars."

Or, in this chapter in a recent book, Overview of Multiple Star Systems, where it states that "Multiple stars represent approximately 50 % of the stellar population..."

The difficulties of establishing accurately the numbers are discussed in depth in Binary and Multiple Star Systems.

My question: do you have an authorative source, or sources to validate your assertion?

 

[Vanadium 50]

http://iopscience.iop.org/article/10.1086/503158

The argument is that most stars are M dwarfs and most M dwarfs are single. If you look at a biased, bright sample, like visible stars, you get a different answer.

 

[CygnusX-1]

The Sun is a main-sequence star, which means it is using nuclear reactions to convert hydrogen into helium at its core. This is the definition of a main-sequence star.

Later in life, the Sun will expand and become a giant. Right now it is much more compact, so to differentiate its current state from its later state, professional astronomers also refer to main-sequence stars as "dwarfs."

However, this term is misleading to laypeople, because it suggests the Sun and all other main-sequence stars are insignificant. In fact, the Sun is more luminous than 95 percent of all stars. Just take a look at the nearest stars for proof.

Even Regulus, a B-type main-sequence star that's more than 100 times as luminous as the Sun, is considered a dwarf. But again, this term does NOT mean that Regulus is dim or insignificant. It's actually one of the most luminous stars within 100 light-years of Earth.

As for being single, most stars are single, because most stars are red dwarfs and most of them are single.

About 50 percent of solar-type stars are single, as stated at the 5-minute mark in this talk, so there's nothing unusual about the Sun's being single.

So, in answer to the original question: Yes, the Sun is a dwarf, but it's much more luminous than most other stars in the Galaxy.

 

[kimbyd]

The Sun is a main-sequence star, which means it is using nuclear reactions to convert hydrogen into helium at its core. This is the definition of a main-sequence star.

Another way to look at it is that main sequence stars behave like balls of ideal gas with fusion at their cores.

Stars evolve away from the main sequence because of quantum effects: the Pauli Exclusion Principle prevents two elections from occupying the same state at the same time. So when the core of the star gets dense enough (e.g. by the accumulation of helium in the core), the Pauli Exclusion Principle starts forcing electrons into higher-energy states to prevent them from being in the same state. This is called degeneracy pressure.

Degeneracy pressure behaves radically differently from the thermal pressure that supports an ideal gas, which dramatically changes the behavior of the star.

 

[Valdis]

This seems to be contentious. For example
Chandra X-Ray Observatory
"More than 80% of all stars are members of multiple star systems containing two or more stars"

Note that isn't incompatible with most stars being singleton systems.

Consider for example a galaxy that has 60% single-star systems, 20% 2-star systems, and 10% each of 3 and 4 star systems.

If you pick 100 systems at random, you'll expect a total of 60 + 2*20 + 3*10 + 4*10 or 170 total stars, of which 110 (or almost 2/3) are in multiple star systems, even though almost 2/3 of the systems are singletons.

 

[rude man]

Earth is a category "G" star.
The categories are, from largest to smallest,"OBAFGKMRNS". So the sun is somewhat above average in size but well below the largest.

(Memorize the sequence as "O B A Fine Girl Kiss Me Right Now Smack".

 

[CygnusX-1]

The Sun is actually FAR above average.

Why? Because K and M dwarfs are far more common than O, B, A, and F stars.

75 percent of all stars are M dwarfs. These red dwarfs are the true average stars, and they are much fainter, cooler, and smaller than the Sun. Or, to put it the other way, the Sun is much brighter, warmer, and larger than the average star in the Galaxy.

 

[rude man]

The Sun is actually FAR above average.

Not in size. Puny compared to O,B,A and F stars.

 

[CygnusX-1]

Yes, the Sun is far above average in size. O, B, A, and F stars account for only about 1 percent of all stars. So what if the Sun is smaller than them? It's larger than M dwarfs, which account for 75 percent of all stars, not to mention K dwarfs and white dwarfs.

 

[stefan r]

Indeed. I believe that all stars larger than "dwarf" stars have passed beyond the main sequence phase, correct?

New stars less than 3 solar mass are larger before they reach the main sequence (hayashi track). Some sorts of collision could also increase luminosity and volume.

Type B and O stars are debatable. The names "dwarf" and "giant" were applied to red stars before fusion was added to the model. One of the trapesium cluster has 10x solar radius and 200,000 luminosity. Still in main sequence but would not have been considered a "dwarf". The convective zone in a large star can extend all the way to center.

Thorne-Zytkow objects and contact binaries are hard to define. A Thorne-Zytkow object that was still main sequence dwarfish would be much more rare than a Thorne-Zytkow object in a giant. It would have a short existence. Is a contact binary 2 stars that are in contact or one object?

Rotation adds a variable. A high rotation star is brighter if you look down on the pole. If "larger" means "brighter per unit of mass" then a high rotation star will be "larger". Not as much if "larger" means "observed radius".

Metallicity effects both size and brightness. More metals means higher density. The higher density increases brightness. So a solar mass population II star is larger than the sun by volume. Some stars closer to the center of the galaxy can have solar mass and higher luminosity.

The effect of rotation and metallicity are minor if you compare solar mass objects. In the hot part of the H-R diagram types III, IV, and V are closer together.

 

[ensign_nemo]

The largest stars burn out far more quickly than their smaller brethren.

https://en.wikipedia.org/wiki/Stellar_evolution

This means that there are relatively few type O, B, or A stars compared to the smaller and dimmer types.

The lifetime of the sun is estimated at 10 billion years (it's middle-aged right now at about 4.5 billion).

The current lifetime of the universe is about 13.7 billion years, so relatively few type G stars have died of old age compared to O, B, A, or F stars.

Any star much smaller than the Sun (K, M, etc.) hasn't had enough time to burn out and die, even if it was born in the first billion years after the Big Bang.

 

[kimbyd]

The largest stars burn out far more quickly than their smaller brethren.

https://en.wikipedia.org/wiki/Stellar_evolution

This means that there are relatively few type O, B, or A stars compared to the smaller and dimmer types.

The lifetime of the sun is estimated at 10 billion years (it's middle-aged right now at about 4.5 billion).

The current lifetime of the universe is about 13.7 billion years, so relatively few type G stars have died of old age compared to O, B, A, or F stars.

Any star much smaller than the Sun (K, M, etc.) hasn't had enough time to burn out and die, even if it was born in the first billion years after the Big Bang.

That's part of the reason. But they're also less common just because it's less common that there is a collapsing gas cloud massive enough to form the more massive stars.

Edit: Note that in young, actively star-forming galaxies, the more massive stars make up a majority of the light coming from those galaxies. The big, massive stars are still not very common, but they are so very much brighter than their less massive peers that they make up most of the light. This is why many very distant galaxies appear blue.

 

[stefan r]

...This means that there are relatively few type O, B, or A stars compared to the smaller and dimmer types...
Any star much smaller than the Sun (K, M, etc.) hasn't had enough time to burn out and die, even if it was born in the first billion years after the Big Bang.

...they are so very much brighter than their less massive peers that they make up most of the light. This is why many very distant galaxies appear blue.

The brightness is fairly close in the milky way too. Sources can be 1% as frequent per unit volume if they shine 100 times brighter. An OBA star can be much further away and still be visible. You can see Deneb at 1500 light years. There are no F or G stars at 1500 light years that you can see without telescopes.