12 votes

Is Pluto still a dwarf planet?

Yes, Pluto is still a dwarf planet. According to the IAU website, it still fits the criteria for a dwarf planet, fails to meet the criteria for a planet, and still carries the "dwarf planet" label, ...
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  • 33.7k
11 votes

Why are type 1b and type 1c supernovae called type 1 rather than type 2; if they result from large exploding stars, rather than accreting dwarfs?

It is fundamentally a question of spectral lines. Type I supernovae have no hydrogen lines, and type II have strong hydrogen lines. Type 1b have strong helium lines and no hydrogen lines, and type 1c ...
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  • 1,987
9 votes

Up to 384 minor planets (including Pluto) qualify as planets?

That actual IAU Resolution B5 adopted at the IAU General Assembly in 2006 states: (1) A planet is a celestial body that: (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity ...
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  • 7,135
8 votes

Sun's Stellar Class at Birth?

Spectral type is determined by (and is an indicator of) photospheric temperature and surface gravity. The classification of G2V applies to a star with a photospheric temperature of about 5800K and a ...
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  • 114k
8 votes

Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?

The original Hertzsprung-Russell diagrams constructed by Henry Russell and Eijnar Hertzsprung consisted of absolute magnitude on the y-axis and a spectral type or an indicator of spectral type on the ...
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  • 114k
7 votes
Accepted

Planets classification by density

There is no 1:1 mapping between density and composition/structure. You have to look at detailed planetary models. For example, some hot Jupiters are extremely dense ($\geq 10$ g/cm$^3$) but they are ...
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  • 114k
7 votes
Accepted

Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?

Originally, what was plotted was luminosity against colour, and by colour I mean the wavelength of the peak intensity. Short wavelengths on the left, and long on the right, as you would expect. Now ...
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  • 87.6k
6 votes

Is the point of this paper to say that Pluto's status as a planet should have been handled scientifically, rather than by a vote?

Part of the problem here is semantics. The meaning of "planet" was really poorly defined prior to 2006 (and some argue that it still is). In the early 2000's, this started to become an issue. You ...
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5 votes
Accepted

Currently proposed ways to distinguish between a really big planet and a really small star

The dividing line between star and brown dwarf is the mass at which hydrogen fusion via the pp-chain occurs at its core. A brown dwarf below this mass limit will never reach a sustainable equilibrium ...
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  • 114k
5 votes
Accepted

Star class according to initial mass

The classification of stars using spectral class is a very useful classification when considering the properties of (the atmosphere of) a star at that moment. If you consider the different stages in ...
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  • 3,311
5 votes
Accepted

Is Ceres in the inner solar system or outer solar system?

Inner. The entire asteroid belt is in the Inner Solar System (now). The definition of "Inner" vs. "Outer" is non-arbitrary, based on the current "frost line", approx. 5 a.u. radius. https://en....
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  • 276
4 votes
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Musk's Roadster is now Russell's Teapot?

Small objects in space are hard to detect and characterise. Compare the tesla with another small solar system body: 2015TC25, It is currently about 0.3AU (3/10 of the distance from Earth to the sun) ...
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  • 87.6k
4 votes
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What abbreviations, letters, or symbols do we use to denote highly degenerate stars?

Neutron stars and black holes do not have assigned spectral types since they do not have a measurable optical/IR spectrum - which is the basis for assigning a spectral type. There are many ...
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  • 114k
4 votes
Accepted

Where did the grand design spiral galaxy designation come from?

The term can be traced back to the theoretical work of C.C.Lin in explaining the spirals as due to density waves in galactic discs. A paper from 1970 discusses the "Existence of 'Grand Design'". Note ...
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  • 87.6k
4 votes
Accepted

Classification and discovery of stars

Cepheid variable stars and exoplanets transiting stars have very different light curves (the relation between brightness and time). Exoplanet light curve from NASA: Cepheid variable light curve from ...
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  • 1,234
4 votes

Is the point of this paper to say that Pluto's status as a planet should have been handled scientifically, rather than by a vote?

To respond to the question as is: yes the paper is trying to point out that the concensus was not scientific. However, this claim is not true at all. In order to understand how classifications within ...
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4 votes
Accepted

What does Sa and Sab mean for spiral galaxies?

Capitalization is critical here. (As is knowing which classification system is being used.) The traditional spiral stages (going back to Hubble) are Sa - Sb - Sc; this was a sequence from more ...
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  • 14.7k
4 votes
Accepted

How many Kuiper Belt objects have moons? How do we know this?

"Moons" is a misnomer here, the majority of Kuiper Belt Objects (KBOs) with companions are binaries. Now there are several populations in the Kuiper Belt, characterized by their ...
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4 votes
Accepted

Is the transition between ice giants and Jupiter-like gas giants somewhat fluid?

The distinction between gas giants and ice giants is a distinction that works nicely in our solar system. (And don't forget the distinction between the giant planets and the terrestrial planets, which ...
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  • 27.1k
3 votes

Is the point of this paper to say that Pluto's status as a planet should have been handled scientifically, rather than by a vote?

If they wanted to make this paper about Pluto, why would they not just discuss Pluto? You may be right that the comment about voting could be linked to the Pluto debate, but I don't see any reason ...
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  • 1,234
3 votes

Currently proposed ways to distinguish between a really big planet and a really small star

Like in each of the other cases you mention, when dividing really big planets from really small stars you're drawing a bright line across a continuous range of objects. Whenever you do that, you're ...
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  • 7,370
3 votes

Is there a strict chronological order that determines the shape of galaxies?

Galaxies evolve from spiral to elliptical. The spirals are formed by patterns of new star formation in the disk surrounding the bulge, which contains mostly old stars. As galaxies run out of ...
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  • 87.6k
3 votes

Betelgeuse and sun classification

If you take a look at the Hertzsprung-Russell Diagram that helps us to classify stars (c) Wikimedia commons (while this one uses data gathered from the Hipparcos satellice, roughly 100.000 stars) ...
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3 votes

Are there ways to determine a star's spectral type without the spectrum itself?

The color index of the star can give you that information. I can look it up for you if I know which exoplanet you're interested in, or you can find it in the NASA Exoplanet Database here.
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2 votes

Classification of a Comet

You've answered your own question there, the difference between the two is that asteroids are rock whereas comets are made up of ice and dust. An asteroid doesn't heat up when it gets close to the ...
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2 votes

How accurate and complete is the Hubble sequence diagram?

The only way the Hubble sequence is truly incomplete is that it assumes such a classification applied to all stages of the universe. We now have telescopes that can see much further in space—and ...
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  • 33.7k
2 votes

Grammar of stellar classification

Normally, you would write G2V (with no spaces), as separating each part of the classification would cause some ambiguity. Basically, any star's classification would be written as 'Temperature class'+'...
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