Stellar Temperature Calculator

Calculate the surface temperature of stars based on their color, spectral class, or luminosity. Understand the relationships between stellar temperature, color, and classification in astrophysics.

Calculation Method

Input Value

Surface temperature in Kelvin

About Stellar Temperature Calculator

Understanding Stellar Temperatures

Stellar temperature refers to the surface temperature of a star, typically measured in Kelvin (K). A star's temperature is one of its most fundamental properties, directly influencing its color, spectral characteristics, and many other observable features. Stars span an impressive temperature range, from the coolest red dwarfs at around 2,000 K to the hottest blue giants exceeding 50,000 K.

Unlike planets, stars are massive balls of plasma where nuclear fusion occurs in their cores. This fusion process generates immense energy that radiates outward and eventually reaches the star's photosphere (visible surface), creating the temperature we observe. The Sun, an average G-class star, has a surface temperature of approximately 5,800 K.

Temperature Determines:

The star's color (from red to blue-white)
Spectral classification (O, B, A, F, G, K, M, L, T, Y)
The types of absorption lines visible in its spectrum
Peak wavelength of emitted radiation (Wien's Law)
Total energy output per unit area (Stefan-Boltzmann Law)

The Spectral Classification System

Astronomers classify stars using the Harvard spectral classification system, which organizes stars primarily by their surface temperature. This system uses the letters O, B, A, F, G, K, M (from hottest to coolest) for main sequence stars, with newer classes L, T, and Y added for ultracool brown dwarfs and sub-stellar objects.

Spectral Classes and Temperatures

Class	Temperature (K)	Color	Example
O	≥ 30,000	Blue	Zeta Ophiuchi
B	10,000 - 30,000	Blue-white	Rigel
A	7,500 - 10,000	White	Sirius
F	6,000 - 7,500	Yellow-white	Procyon
G	5,200 - 6,000	Yellow	Sun
K	3,700 - 5,200	Orange	Arcturus
M	2,400 - 3,700	Red	Betelgeuse
L	1,300 - 2,400	Deep red	Teide 1
T	550 - 1,300	Methane brown	WISE 0855-0714
Y	< 550	Infrared only	WISE 1828+2650

The Physics of Star Temperature

Wien's Displacement Law

Wien's displacement law describes the relationship between a star's temperature and the peak wavelength of its emission spectrum:

λ_max = b/T

Where λ_max is the peak wavelength, T is the temperature in Kelvin, and b ≈ 2.898×10^-3 m·K is Wien's displacement constant

This explains why hotter stars appear blue (peak emission at shorter wavelengths) and cooler stars appear red (peak emission at longer wavelengths).

Stefan-Boltzmann Law

The Stefan-Boltzmann law relates a star's surface temperature to its energy output per unit area:

F = σT⁴

Where F is the energy flux, T is the temperature in Kelvin, and σ ≈ 5.67×10^-8 W·m^-2·K^-4 is the Stefan-Boltzmann constant

This powerful relation shows that doubling a star's temperature increases its energy output by 16 times, explaining the extreme luminosity of hot stars.

Measuring Stellar Temperatures

Astronomers employ several sophisticated methods to determine the temperatures of distant stars. Each technique has advantages and limitations, and often multiple methods are used together for more accurate results.

Spectroscopic Methods

Analysis of absorption line strengths
Ionization balance of different elements
Hydrogen line profiles (especially Balmer lines)
Molecular band strengths (for cooler stars)
Line ratio techniques (temperature-sensitive line pairs)

Photometric Methods

Color index measurements (B-V, U-B, etc.)
Fitting to blackbody curves
Infrared flux method (IRFM)
Surface brightness techniques
Interferometric angular diameter measurements

Temperature and Stellar Evolution

A star's temperature evolves throughout its lifetime, reflecting its internal nuclear processes. Most stars begin their main sequence lives with temperatures determined primarily by their mass - more massive stars have higher core pressure and temperature, resulting in higher surface temperatures.

Evolution of Stellar Temperature

Protostars

Initially cool (few thousand K) and reddish, gradually warming as they contract

Main Sequence

Stable temperature for billions of years; hotter for massive stars (O/B-type), cooler for low-mass stars (K/M-type)

Red Giant Phase

Core heats up while surface cools (3,000-4,000 K) and expands

Final Stages

- White dwarfs: Very hot initially (20,000+ K) but cool over billions of years
- Neutron stars: Extremely hot (millions of K) at formation, cooling over time
- Black holes: No temperature in classical sense, but accretion disks can reach millions of K

Frequently Asked Questions

What is stellar temperature?

Stellar temperature refers to the surface temperature of a star, typically measured in Kelvin (K). This temperature is determined by the nuclear fusion reactions occurring in the star's core and affects its color, spectral classification, and many other observable properties. Surface temperatures of stars range from around 2,000 K for the coolest red dwarfs to over 50,000 K for the hottest blue giants.

How is stellar temperature related to star color?

Stellar temperature directly determines a star's color due to blackbody radiation principles. The hottest stars appear blue or blue-white because they emit more energy at shorter (bluer) wavelengths. Intermediate-temperature stars like our Sun appear yellow or white. Cooler stars appear orange or red as they emit more energy at longer (redder) wavelengths. This relationship is described by Wien's displacement law, which states that the peak wavelength of emission is inversely proportional to the star's temperature.

What is the Harvard spectral classification system?

The Harvard spectral classification system categorizes stars based on their spectral characteristics, which correlate with surface temperature. The main classes are O, B, A, F, G, K, and M (often remembered with the mnemonic "Oh Be A Fine Girl/Guy, Kiss Me"), arranged from hottest to coolest. Each class is further subdivided with numbers from 0-9 (e.g., G2). More recently, classes L, T, and Y have been added to accommodate brown dwarfs and other ultracool objects.

How do astronomers measure the temperature of distant stars?

Astronomers determine stellar temperatures through several methods: spectroscopy (analyzing the absorption lines in a star's spectrum), color index measurements (comparing brightness in different wavelength bands), and Wien's displacement law (finding the peak wavelength of the star's emission). For some nearby stars, interferometry can directly measure their size, which combined with their total luminosity, allows temperature calculation using the Stefan-Boltzmann law.

What is the relationship between a star's temperature and its lifespan?

Generally, hotter stars have shorter lifespans while cooler stars live longer. This occurs because hotter, more massive stars burn their nuclear fuel more rapidly due to higher core temperatures and pressures. For example, hot O-class stars (30,000+ K) may live only a few million years, while our G-class Sun (5,800 K) will live about 10 billion years, and the coolest M-class red dwarfs (under 3,500 K) can potentially shine for trillions of years. This inverse relationship between temperature and lifespan is a fundamental concept in stellar evolution.

Additional Resources

NASA: Stellar Spectra Harvard: Stellar Evolution

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