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Starts With A Bang

Mass means (almost) everything in astronomy

What kind of object will you form? What will its fate be? How long will a star live? Almost everything is determined by mass alone.
planetary nebulae infrared spitzer
These three planetary nebulae, all imaged by Spitzer, highlight features inherent to dying Sun-like stars. From left to right, the Exposed Cranium Nebula, the Ghost of Jupiter Nebula, and the Little Dumbbell Nebula all exhibit stellar winds, ejected material consisting of different elements, and a central, luminous stellar remnant. Only objects within a specific mass range will experience this phenomenon as their ultimate fate.
(Credit: NASA/JPL-Caltech)
Key Takeaways
  • In our Universe, there are all sorts of properties that one can measure about an object: mass, surface gravity, number of particles, its relative composition, the volume it occupies, etc.
  • But if you want to know what your object is going to be, look like, and how it’s going to behave over its lifetime, mass is a far more important factor than anything else.
  • Here’s where the (rough) dividing lines are between objects of different sizes in astronomy, and why mass matters so much.

The Universe is full of variety.

NGC 2014
This one small region near the heart of NGC 2014 showcases a combination of evaporating gaseous globules and free-floating Bok globules, as the dust goes from hot, tenuous filaments at top to denser, cooler clouds where new stars form inside below. The mix of colors reflects a difference in temperatures and emission lines from various atomic signatures. This neutral matter reflects starlight, where this reflected light is known to be distinct from the cosmic microwave background.
(Credit: NASA, ESA and STScI)

From individual particles to ultramassive black holes, the Universe contains it all.

galactic center spitzer
This three-color composite shows the galactic center as imaged in three different wavelength bands by NASA’s Spitzer: the predecessor to the James Webb Space Telescope. Carbon-rich molecules, known as polycyclic aromatic hydrocarbons, show up in green, while stars and warm dust are also visible. A glow where our supermassive black hole sits is identifiable as well. The presence of ethyl formate was found in the gas cloud Sagittarius B2: the same molecule that gives raspberries their characteristic scent.
(Credit: NASA/JPL-Caltech)

All bound structures possess many physical properties.

supermassive black hole m87*
The second-largest black hole as seen from Earth, the one at the center of the galaxy M87, is shown in three views here. At the top is optical from Hubble, at the lower-left is radio from NRAO, and at the lower-right is X-ray from Chandra. These differing views have different resolutions dependent on the optical sensitivity, wavelength of light used, and size of the telescope mirrors used to observe them. These are all examples of radiation emitted from the regions around black holes, demonstrating that black holes aren’t so black, after all.
(Credit: Optical: Hubble/NASA/Wikisky; Radio: NRAO/Very Large Array; X-ray: NASA/Chandra/CXC)

Mass, alone, can roughly determine their natures.

cigar galaxy messier 82
This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds. The lower in mass the wind-driven starburst galaxy is, the more of its normal matter it loses during an event like this.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)

Individual atoms are minuscule: between 10-30 and 10-28 grams.

JWST spectroscopy composition star forming region
As spectroscopic imaging with JWST reveals, chemicals like atomic hydrogen, molecular hydrogen, and hydrocarbon compounds occupy different locations in space within the Tarantula Nebula, showcasing how varied even a single star-forming region can be. Atoms, ions, and molecules all exist throughout the cosmos.
(Credit: NASA, ESA, CSA, STScI, Webb ERO Production Team)

They combine, forming heavier molecules, typically up to ~10-24 grams.

interstellar molecules
The existence of complex, carbon-based molecules in star forming regions is interesting, but isn’t anthropically demanded. Here, glycolaldehydes, an example of simple sugars, are illustrated in a location corresponding to where they were detected in an interstellar gas cloud: offset from the region presently forming new stars the fastest. Interstellar molecules are common, with many of them being complex and long-chained.
Credit: ALMA (ESO/NAOJ/NRAO)/L. Calçada (ESO) & NASA/JPL-Caltech/WISE Team

Various molecules bind together, forming dust grains beginning at ~10-14 grams.

bok globule
Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked nearly as much, as the smaller-sized dust grains (down to about half-a-micron across) are too little to interact with the long-wavelength light. At longer wavelengths, more of the Universe beyond the light-blocking dust can be revealed.
(Credit: ESO)

Bigger grains make larger irregular “clumps,” up to masses of ~1019 kilograms.

itokawa composition
A schematic view of the strange peanut-shaped asteroid Itokawa. Itokawa is an example of a rubble-pile asteroid, but determinations of its density have revealed that it is likely a result of a merger between two bodies that have different compositions. It lacks the necessary mass/gravity to pull itself into a round shape.
(Credit: ESO, JAXA)

Above that, however, objects reach hydrostatic equilibrium.

Mimas, as imaged here during the closest fly-by of Cassini in 2010, is only 198 kilometers in radius, but is quite clearly round owing to its self-gravitation. Being made mostly of ice, it does what the larger asteroids Vesta and Pallas cannot: pull itself into a spheroidal shape. However, many argue over whether it’s truly in hydrostatic equilibrium, as the large crater visible here, Herschel, might not persist if the world were truly shaped by self-gravitation.
(Credit: NASA/JPL-Caltech/Space Science Institute)

Ice-rich objects become spheroidal at ~3 × 1019 kg, while rocky/metallic objects require ~3 × 1020 kg.

rocky planets moons KBOs
Although Earth and Venus are the two largest rocky objects in the Solar System, Mars, Mercury, as well as over 100 of the largest moons, asteroids, and Kuiper belt objects have all achieved hydrostatic equilibrium.
(Credit: Emily Lakdawalla. Data from NASA / JPL, JHUAPL/SwRI, SSI, and UCLA / MPS / DLR / IDA, processed by Gordan Ugarkovic, Ted Stryk, Bjorn Jonsson, Roman Tkachenko, and Emily Lakdawalla)

They’ll remain solid-surfaced until exceeding ~1025 kg: about double Earth’s mass.

most earth like world
The eight most Earth-like worlds, as discovered by NASA’s Kepler mission: the most prolific planet-finding mission to date. All of these planets orbit stars smaller and less bright than the Sun, and all of these planets are larger than Earth, with many of them likely possessing volatile gas envelopes. Although some of them are called super-habitable in the literature, we don’t yet know if any of them have, or ever had, life on them at all, but the border between “rocky” and “gas-rich” is still being studied, and most or even all of these selected Kepler planets may yet have volatile gas envelopes around them.
Credit: NASA Ames/W Stenzel

Above that, objects become gas-rich, like Neptune/Saturn, up to ~1027 kg.

solar system worlds
By size, it’s clear that the gas giant worlds vastly outstrip any of the terrestrial planets. In terms of temperature, the distance from the parent star is the overwhelming factor for a planet’s temperature so long as it doesn’t produce much of its own internal heat. In our Solar System, a Pluto-like object is at ~40 K, while Venus is the hottest planet at ~700+ K.
Credit: CactiStaccingCrane/Wikimedia Commons

The heaviest planets achieve Jupiter-like self-compression: up to ~2-3 × 1028 kg.

When we classify the known exoplanets by both mass and radius together, the data indicates that there are only three classes of planets: terrestrial/rocky, with a volatile gas envelope but no self-compression, and with a volatile envelope and also with self-compression. Anything above that becomes first a brown dwarf and then a star. Planetary size peaks at a mass between that of Saturn and Jupiter, although there are a few “puffy” super-Jupiters, with a likely unusually light composition.
Credit: J. Chen and D. Kipping, ApJ, 2017

Above that deuterium fusion begins, creating a brown dwarf star.

brown dwarf ESO
The exoplanet Kepler-39b is one of the most massive ones known, at 18 times the mass of Jupiter, placing it right on the border between planet and brown dwarf. In terms of radius, however, it’s only 22% larger than Jupiter, as deuterium fusion doesn’t substantially change the self-compressed object’s size. Objects up to ~80 times the mass of Jupiter are still approximately the same size.
Credit: ESO

At 1.5 × 1029 kg, hydrogen fusion occurs, indicating a full-fledged star.

morgan keenan spectral classification
The (modern) Morgan–Keenan spectral classification system, with the temperature range of each star class shown above it, in kelvin. In terms of size, the smallest M-class stars are still about 12% the diameter of the Sun, but the largest main sequence stars can be dozens of times the Sun’s size, with evolved red supergiants (not shown) reaching hundreds or even 1000+ times the size of the Sun. A star’s (main sequence) lifetime, color, temperature, and luminosity are all primarily determined by a single property: mass.
Credit: LucasVB/Wikimedia Commons; Annotations: E. Siegel

Stars born above ~8 × 1029 kg evolve into planetary nebula/white dwarf combinations.

planetary nebula
When our Sun runs out of fuel, it will become a red giant, followed by a planetary nebula with a white dwarf at the center. The Cat’s Eye nebula is a visually spectacular example of this potential fate, with the intricate, layered, asymmetrical shape of this particular one suggesting a binary companion. At the center, a young white dwarf heats up as it contracts, reaching temperatures tens of thousands of Kelvin hotter than the surface of the red giant that spawned it. The outer shells of gas are mostly hydrogen, which gets returned to the interstellar medium at the end of a Sun-like star’s life.
Credit: Nordic Optical Telescope and Romano Corradi (Isaac Newton Group of Telescopes, Spain)

Stars above ~2 × 1031 kg go supernova, becoming neutron stars or black holes.

crab pulsar remnant
A combination of X-ray, optical, and infrared data reveal the central pulsar at the core of the Crab Nebula, including the winds and outflows that the pulsars carry in the surrounding matter. The central bright purplish-white spot is, indeed, the Crab pulsar, which itself spins at about 30 times per second. The material shown here spans about 5 light-years in extent, originating from a star that went supernova about 1,000 years ago, teaching us that the typical speed of the ejecta is around 1,500 km/s. The neutron star originally reached a temperature of ~1 trillion K, but even now, it’s already cooled to “only” about 600,000 K.
Credit: X-ray: NASA/CXC/SAO; Optical: NASA/STScI; Infrared: NASA-JPL-Caltech

More massive stellar remnants always remain black holes, without upper mass limits.

OJ 287
This diagram shows the relative sizes of the event horizons of the two supermassive black holes orbiting one another in the OJ 287 system. The larger one, of ~18 billion solar masses, is 12 times the size of Neptune’s orbit; the smaller, of 150 million solar masses, is about the size of the asteroid Ceres’s orbit around the Sun. The heaviest known black hole is only a few times more massive (and hence, a few times larger in radius) than OJ 287’s primary.
(Credit: NASA/JPL-Caltech/R. Hurt (IPAC))

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


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