# Questions tagged [classification]

Questions regarding grouping objects together based on one or more shared property.

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### What does Sa and Sab mean for spiral galaxies?

I have learned that the notation to classify spiral galaxies in the De Vaucouleurs system goes like "SA for spirals without bar, SB for spirals with bar and SAB for intermediate spirals". But in some ...
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### Why have brown dwarf classes been dubbed L, T and Y?

The classes used to categorize stars (O, B, A, F, G, K, M) are in a bizarre order for historical reasons. Stars were labeled based on the spectral lines that were visible, then the categories were put ...
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### How are Y-dwarf spectral subtypes defined?

It is often stated that the spectral subtype indicates the relative position between spectral types, e.g. G5 being halfway from the start of spectral type G to the beginning of spectral type F. Given ...
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### Grammar of stellar classification

How to write G2V correctly (I mean space)? I saw in articles $G\,2\,V$ and $G2\,V$, on the internet webside it is usually $G2V$.
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### Can Jupiter's nightside be classified as spectral type Y?

As far as I am aware, the latest spectral types that have been assigned are around Y2, for objects like WISE 0855-0714 that have temperatures around 250 K or so. I've also seen several directly-imaged ...
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### Sun's Stellar Class at Birth?

The Sun currently has a stellar class of G2V. However, since its birth, the Sun has brightened significantly. What would the Sun's class have been when it was born ~4.6 gyr ago?
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### 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?

The abstract of a paper currently in-press in Icarus: The Reclassification of Asteroids from Planets to Non-Planets (Metzger et al. 2018, https://doi.org/10.1016/j.icarus.2018.08.026) says: It is ...
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### Classification and discovery of stars

How do astronomers find the difference between a 'Cephid Variable Star' and distinct normal stars with opaque objects blocking it's light, like stars with planets revolving around them while both ...
445 views

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

The Next Big Future article Rogue Exoplanet 12.7 times bigger than Jupiter is 20 light years away Astronomers using the National Science Foundation’s Karl G. Jansky Very Large Array (VLA) have made ...
121 views

### Musk's Roadster is now Russell's Teapot?

Space Exploration Stack Exchange were not very fond of this question and closed it, so I think it might fit better here. Summary: Assuming no foreknowledge of its existence, can Elon Musk's Tesla ...
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### What abbreviations, letters, or symbols do we use to denote highly degenerate stars?

We have OBAFGKM to denote the strength of hydrogen lines. Apparently we added S, N, C, and the W classes when we learned of new kinds of stars. We use T, L, and Y to denote brown dwarfs. The D ...
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### How are the numerical stellar subclassifications determined?

From Wikipedia: The spectral classes O through M, as well as other more specialized classes discussed later, are subdivided by Arabic numerals (0–9), where 0 denotes the hottest stars of a given ...
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### Is there an alternative (potentially easier to understand) Euler diagram of the new solar system body classification?

note: This question is asking for a representation of the astronomical classification scheme now used for solar system bodies as established (presumably) by the Working Group for Planetary System ...
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### How accurate and complete is the Hubble sequence diagram?

The Hubble sequence sorts and characterizes or classifies galaxies by their shape. It was suggested by Edwin Hubble back in 1926. One drawback, or criticism, of this scheme is that Hubble had a ...
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### Where did the grand design spiral galaxy designation come from?

What is the history behind the name that we gave to that class of galaxies?
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### Is there a strict chronological order that determines the shape of galaxies?

Assuming no collision happens, do all elliptical galaxies eventually become spiral? also when a galaxy becomes spiral or barred spiral, can it switch between these shapes or is it set forever?
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### Is Ceres in the inner solar system or outer solar system?

Do we consider the dwarf planet Ceres to be in the inner solar system or outer solar system (or possibly neither / in between). It is located within the asteroid belt, but is the asteroid belt part ...
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### Planets classification by density

In our Solar system, the density of major planets varies from 0.7 g/cm3 (Saturn) to 5.5 g/cm3 (Earth). For exoplanets, the density varies from very low (0.03 g/cm3 for Kepler-51x) to very high (77.7 ...
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### Betelgeuse and sun classification

In this video, between 4 and 6 minutes, the lecturer describes M type stars as being less massive, smaller, and redder than G type stars. He then gives Betelgeuse as an example of an M type star and ...
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### Why does spectral class and U-V colour correlate slightly differently for main sequence, giant and supergiant stars?

In this diagram the $x$-axis is spectral type for stars and the $y$-axis is B-V colour. Taking Series 1 as the Main Sequence, Series 2 as Giants and Series 3 as Super-giants. Why the spectral class ...
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### Why does the Hertzsprung–Russell diagram's x-axis go from large temperatures to lower?

In my textbook, the H–R diagram’s y-axis is $\log(L/L_{\odot})$ and values are higher as we go higher on the scale, but the x-axis is $\log(T_\text{eff})$ and gets smaller as we proceed to the right. ...
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### Synthesising types of galaxies using various stellar spectra

I have asked this on Physics Stack Exchange but Astronomy is probably more appropriate. I have been given the task of synthesising an elliptical galaxy, a starburst galaxy and a spiral galaxy (...
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### Star class according to initial mass

Studying the stars classes for my homework (school) i learned that stars are classified by the spectral types (Harvard) and luminosity class (MKK) .. So the sun is a star class ...
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### Is Pluto still a dwarf planet?

Recent news seem to suggest that astronomers are arguing about whether Pluto should become a planet again. However, I cannot find an official source for this. Is this actually true?
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### Classification of a Comet

Google's definition of a comet is: a celestial object consisting of a nucleus of ice and dust and, when near the sun, a ‘tail’ of gas and dust particles pointing away from the sun. This doesn'...
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### Our galaxy, the Milky Way, forms part of a galaxy group called The Local Group. What are the parameters that dictate what a galaxy group is?

Are there different classifications or just the one? How does our galaxy group compare with others?