Thor's Helmet

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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 Rosetta Nebula and NGC 2244

At the heart of the Rosetta nebula is an open cluster of stars, which is NGC 2244. The entire structure sits at the end of a giant molecular cloud, and you can easily spot the tell tale columns and protostars at the head showing active star formation on-going.

The bottom image is from Chandra, which gives the purple/pink colours for X-Ray sources, used particularly in spotting black hole accretion disks, pulsars and supernova remnants, we can see it's a lively place !

The entire structure is in the constellation of Monoceros at around 5,200 light years from Earth.

Exploration is in our nature. We began as wanderers, and we are wanderers still.

l photo: Sara Hunt l quote: Carl Sagan

Jupiter's Galilean Moons: Io, Europa, Ganymede, Callisto (x)

Over 800 terrestrial exoplanets visualized and arranged according to their equilibrium temperature and size.

chart by u/mVargic

The close-up of the Andromeda Galaxy from the Hubble Space Telescope shows how many stars there really are.

source

Andromeda over the Swiss Alps Image Credit: Dzmitry Kananovich

The four giant planets of our Solar System, as seen by NASA's James Webb Telescope.

Venus makes Oxygen During the Day

Ok, it's little more than an excuse to show a beautiful image of a fascinating planet, but in a recent scientific study, it was discovered in the upper atmosphere of Venus, during the day (which lasts a little longer than our day, at 243 Earth Days !!) Carbon Dioxide and Carbon Monoxide is broken down by sunlight into it's Carbon and Oxygen. The oxygen then moves around the planet, and over to the night side too, where it plays an important role in the upper atmosphere of the planet.

Amusingly, a full year on Venus is only 225 Earth days, making a day on Venus longer than a year on Venus.

I guess the upshot of this is, every day is your Birthday on Venus, so Happy Venusian Birthday to you all !