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Starts With A Bang

Recipe for how to kill a galaxy revealed at last

With hundreds of billions of stars burning bright, some galaxies are already dead. Their inhabitants might not know it, but we're certain.
NGC 1277 red and dead
This nearby galaxy, NGC 1277, although it may appear similar to other typical galaxies found in the Universe, is remarkable for being composed primarily of older stars. Both its intrinsic stellar population and its globular clusters are all very red in color, indicating that it hasn't formed new stars in ~10 billion years.
Credit: NASA, ESA, and M. Beasley (Instituto de Astrofísica de Canarias)
Key Takeaways
  • When the last remnants of molecular gas, the key ingredient behind the formation of new stars, has been removed from a galaxy, it ceases to meet the astronomical definition of "alive."
  • Although the already-formed stars may continue living for billions or even trillions of years, no new stars will be created. Such galaxies are known as "red and dead."
  • Right in our own cosmic backyard, a well-studied galaxy, NGC 1277, has shown that it died, in this sense, many billions of years ago. Every galaxy will share this fate, including, someday, our own.

Most observed galaxies, astronomically, are alive.

Hickson Compact Group 40 galaxies
While there are many instances of numerous galaxies in the same region of space, they normally occur either between two galaxies only or in very dense regions of space, like at the centers of galaxy clusters. Seeing 5 galaxies interacting within a space of less than 1 million light-years is an extreme rarity, captured in gorgeous detail by Hubble here. As all of these galaxies are still forming new stars, they’re all classified as “alive” by astronomers.
Credit: NASA, ESA, STScI; Processing: Alyssa Pagan (STScI)

Alive, to a galaxy, means “actively forming stars.”

Southern Pinwheel M83
The Southern Pinwheel Galaxy, Messier 83, displays many features common to our Milky Way, including spiral arms and a central bar, as well as spurs and minor arms. The pink regions showcase transitions in hydrogen atoms driven by ultraviolet light. Since that light is primarily produced by hot, blue stars, it’s only the regions where new star-formation is actively occurring where those pink features appear.
Credit: CTIO/NOIRLab/DOE/NSF/AURA; Acknowledgment: M. Soraisam (University of Illinois); Image processing: Travis Rector (University of Alaska Anchorage), Mahdi Zamani & Davide de Martin

Several large star-forming regions line our Milky Way’s spiral arms.

Eagle nebula wide field
This ground-based, wide-field image of the Eagle Nebula shows the star-forming region in all its glory, with new stars, reflection and emission nebulae, and dusty features all present. Note how the material around the stars gets ionized and over time becomes transparent to all forms of light. Star-forming regions in the Milky Way are few in number and small in nature, particularly in comparison to the more active galaxies in our Universe.
Credit: ESO

When new stars form, they arrive with various masses and colors.

morgan keenan spectral classification stars
The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. The overwhelming majority (80%) of stars today are M-class stars, with only 1-in-800 being an O-class or B-class star massive enough for a core-collapse supernova. Our Sun is a G-class star, unremarkable but brighter than all but ~5% of stars, by number. Earlier on, when there were no heavy elements, virtually all of the stars that formed were O-and-B stars: the hottest, bluest, most massive type.
Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel

Although they all form simultaneously, the hottest, bluest, shortest-lived stars evolve and die first.

Tarantula Nebula JWST stars
The central concentration of this young star cluster found in the heart of the Tarantula Nebula is known as R136, and contains many of the most massive stars known. Among them is R136a1, which comes in at about ~260 solar masses, making it the heaviest known star. Although great numbers of cooler, redder stars are also present, the brightest, bluest ones dominate this image.
Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

A few galaxies — mostly ellipticals within clusters — ceased forming stars long ago.

Perseus cluster chain of galaxies
This chain of large galaxies is found near the center of the Perseus cluster of galaxies, with several of these galaxies being typical of the large, bright, evolved galaxies found at the centers of most massive galaxy clusters. For many of these galaxies, the stars found inside of them are primarily older and redder, with only small populations of bluer stars found inside.
Credit: NASA, ESA, and M. Beasley (Instituto de Astrofísica de Canarias)

Over time, the heavier, bluer stars die off.

globular star cluster age
By mapping out the colors and magnitudes of stars that were all born at the same time, like members of a star cluster or globular cluster, you can determine the age of the cluster by identifying where the main sequence ends and the heavier, more massive stars have “turned off” and begun evolving into subgiants. Measuring the subgiant population very well is one key to understanding a stellar population’s age.
Credit: Mike Guidry, University of Tennessee

Since redder stars survive, galaxies lacking new stellar populations are called “red-and-dead” by astronomers.

Abell S740 galaxy ESO 325-G004
Galaxy clusters, like Abell S740, are the largest bound structures in the Universe. When spirals merge, for example, a large number of new stars form, but either post-merger or by speeding through the intra-cluster medium, gas can be stripped away, leading to the end of star formation in that galaxy and, eventually, a red-and-dead final structure.
Credit: NASA, ESA, Hubble Heritage Team (STScI / AURA); J. Blakeslee

Measuring the intrinsic color of a galaxy’s starlight determines whether a galaxy is dead or alive.

galaxy cluster colors
In this image, a massive set of galaxies at the center causes many strong lensing features to appear. Background galaxies have their light bent, stretched, and otherwise distorted into rings and arcs, where it gets magnified by the lens as well. This gravitational lens system is complex, but informative for learning more about Einstein’s relativity in action.
Credit: ESA, NASA, K. Sharon (Tel Aviv University) and E. Ofek (Caltech)

Mainstream astronomy suggests that molecular gas reserves form new stars within galaxies.

Cartwheel galaxy new star formation
This image features data from 10 different JWST filters: 6 from the near-infrared and 4 from the mid-infrared. As a result, features that include stars, gas, dust, and various molecular signatures can all be revealed at once, showcasing where star formation is occurring and will occur in the future. Molecular gas, present in great abundance, is the key.
Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team

If a galaxy possesses no new stars, it must be gas-free.

Perseus cluster NGC 1277 highlight
The ‘red-and-dead’ galaxy NGC 1277 is found inside the Perseus cluster. While the other galaxies contain a mix of red-and-blue stars, this galaxy hasn’t formed new stars in approximately 10 billion years.
Credit: NASA, ESA, M. Beasley (Instituto de Astrofísica de Canarias), and P. Kehusmaa

Gas can be removed by intense star-formation periods, frequently triggered by mergers and interactions.

Cigar Galaxy M82
Galaxies undergoing massive bursts of star formation expel large quantities of matter at great speeds. They also glow red, covering the whole galaxy, thanks to hydrogen emissions. This particular galaxy, M82, the Cigar Galaxy, is gravitationally interacting with its neighbor, M81, causing this burst of activity.
Credits: NASA, ESA and the Hubble Heritage Team (STScI/AURA); Acknowledgment: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation)

Rapid journeys through a galaxy cluster’s intergalactic medium also strip interior gas away.

runaway galaxy pressure stripping
Located within the Norma cluster of galaxies, ESO 137-001 speeds through the intracluster medium, where interactions between the matter in the space between galaxies and the rapidly-moving galaxy itself cause ram pressure-stripping, leading to a new population of tidal streams and intergalactic stars. Sustained interactions such as this can eventually remove all of the gas from within a galaxy, but also teach us that the galaxy, the cluster, and the gas within it is all made of matter, not antimatter.
Credit: NASA, ESA, CXC

In 2018, the first red-and-dead galaxy in our cosmic backyard was identified: NGC 1277.

Speeding at 900 km/s through the Perseus cluster, it hasn’t formed new stars in ~10 billion years.

galaxy NGC 1277 Perseus cluster
The galaxy NGC 1277, speeding through the Perseus cluster, not only contains predominantly red stars, but red (and not blue) globular clusters, as well as an abnormally large supermassive black hole to go along with its rapid speed through this rich galaxy cluster.
Credit: M.A. Beasley et al., Nature, 2018

Its stars and globular clusters are exclusively red-colored.

NGC 1277 and NGC 1278 Perseus cluster
This is a blink comparison that plots the location of the red stars and blue stars that dominate the globular clusters in galaxies NGC 1277 and NGC 1278. It shows that NGC 1277 is dominated by ancient red globular clusters. This is evidence that galaxy NGC 1277 stopped making new stars many billions of years ago, compared to NGC 1278, which has more young blue star clusters.
Credit: NASA, ESA, and Z. Levay (STScI)

Unless gas reserves arrive anew, no new stars should form within it.

globular clusters sorted by color
This graph shows the distribution of globular clusters as sorted by the color of the stars inside, for the field of view of the Perseus Cluster that includes the neighboring galaxies NGC 1277 and NGC 1278. While NGC 1278 and background globular clusters are predominantly blue, NGC 1277 shows itself to be red-and-dead. Some studies suggest that no new stars have been formed within it for approximately 10 billion years.
Credit: M.A. Beasley et al., Nature, 2018

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words.


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