2023 September 28

by Daniel Cid

Cosmic Alphabet Soup: Classifying Stars

If you’ve spent much time stargazing, you may have noticed that while most stars look white, some are reddish or bluish. Their colors are more than just pretty – they tell us how hot the stars are. Studying their light in greater detail can tell us even more about what they’re like, including whether they have planets. Two women, Williamina Fleming and Annie Jump Cannon, created the system for classifying stars that we use today, and we’re building on their work to map out the universe.

By splitting starlight into spectra – detailed color patterns that often feature lots of dark lines – using a prism, astronomers can figure out a star’s temperature, how long it will burn, how massive it is, and even how big its habitable zone is. Our Sun’s spectrum looks like this:

Astronomers use spectra to categorize stars. Starting at the hottest and most massive, the star classes are O, B, A, F, G (like our Sun), K, M. Sounds like cosmic alphabet soup! But the letters aren’t just random – they largely stem from the work of two famous female astronomers.

Williamina Fleming, who worked as one of the famous “human computers” at the Harvard College Observatory starting in 1879, came up with a way to classify stars into 17 different types (categorized alphabetically A-Q) based on how strong the dark lines in their spectra were. She eventually classified more than 10,000 stars and discovered hundreds of cosmic objects!

That was back before they knew what caused the dark lines in spectra. Soon astronomers discovered that they’re linked to a star’s temperature. Using this newfound knowledge, Annie Jump Cannon – one of Fleming’s protégés – rearranged and simplified stellar classification to include just seven categories (O, B, A, F, G, K, M), ordered from highest to lowest temperature. She also classified more than 350,000 stars!

Type O stars are both the hottest and most massive in the new classification system. These giants can be a thousand times bigger than the Sun! Their lifespans are also around 1,000 times shorter than our Sun’s. They burn through their fuel so fast that they only live for around 10 million years. That’s part of the reason they only make up a tiny fraction of all the stars in the galaxy – they don’t stick around for very long.

As we move down the list from O to M, stars become progressively smaller, cooler, redder, and more common. Their habitable zones also shrink because the stars aren’t putting out as much energy. The plus side is that the tiniest stars can live for a really long time – around 100 billion years – because they burn through their fuel so slowly.

Astronomers can also learn about exoplanets – worlds that orbit other stars – by studying starlight. When a planet crosses in front of its host star, different kinds of molecules in the planet’s atmosphere absorb certain wavelengths of light.

By spreading the star’s light into a spectrum, astronomers can see which wavelengths have been absorbed to determine the exoplanet atmosphere’s chemical makeup. Our James Webb Space Telescope will use this method to try to find and study atmospheres around Earth-sized exoplanets – something that has never been done before.

Our upcoming Nancy Grace Roman Space Telescope will study the spectra from entire galaxies to build a 3D map of the cosmos. As light travels through our expanding universe, it stretches and its spectral lines shift toward longer, redder wavelengths. The longer light travels before reaching us, the redder it becomes. Roman will be able to see so far back that we could glimpse some of the first stars and galaxies that ever formed.

Learn more about how Roman will study the cosmos in our other posts:

Roman’s Family Portrait of Millions of Galaxies

New Rose-Colored Glasses for Roman

How Gravity Warps Light

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The Seagull Nebula © Gianni Lacroce

Cosmic Dance of Spiral Galaxies: Arp 238

Crab Nebula

Uranus- JWST NIRCam

This image also shows 14 of the planet’s 27 moons: Oberon, Titania, Umbriel, Juliet, Perdita, Rosalind, Puck, Belinda, Desdemona, Cressida, Ariel, Miranda, Bianca, and Portia.

This picture shows Uranus's north polar cap as well as a storm just below the polar edge.

A stellar exodus was caught in action!  Astronomers used the Hubble Space Telescope to watch the white dwarf exodus in the globular star cluster 47 Tucanae, a dense swarm of hundreds of thousands of stars in our Milky Way galaxy. Hubble took snapshots of fledgling white dwarf stars beginning their slow-paced, 40-million-year migration from the crowded center of an ancient star cluster to the less populated suburbs. By observing ultraviolet light, astronomers examined 3,000 white dwarfs, tracing two populations with diverse ages and orbits. One grouping was 6 million years old and had just begun their journey. Another was around 100 million years old and had already arrived at its new homestead far from the center, roughly 1.5 light-years, or nearly 9 trillion miles (14 trillion kilometers), away. The cluster resides 14,500 light-years away in the southern constellation Tucana. Credit: NASA, ESA, and H. Richer and J. Heyl (University of British Columbia, Vancouver, Canada). ALT TEXT: Thousands of stars, seen as tiny dots, are shown on a black background. The stars vary in size and color, including orange, yellow, and white.

Boo! Did we get you? 🎃

This solar jack-o-lantern, captured by our Solar Dynamics Observatory (SDO) in October 2014, gets its ghoulish grin from active regions on the Sun, which emit more light and energy than the surrounding dark areas. Active regions are markers of an intense and complex set of magnetic fields hovering in the sun’s atmosphere.

The SDO has kept an unblinking eye on the Sun since 2010, recording phenomena like solar flares and coronal loops. It measures the Sun’s interior, atmosphere, magnetic field, and energy output, helping us understand our nearest star.

Grab the high-resolution version here.

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The 2nd full moon of August 2023 l Rami Ammoun