On average, the universe today is an extremely cold place.
At every point in our cosmic history, every observer will experience a uniform “bath” of radiant radiation emanating from the Big Bang. Today, from our perspective, it is only 2725 K above absolute zero and is therefore observed as the cosmic microwave background that peaks at microwave frequencies. At large cosmic distances, when we look back in time, this temperature was higher depending on the redshift of the observed distant object.
(Credit: Earth: NASA / BlueEarth; Milky Way: ESO / S. Brunier; WBC: NASA / WMAP)
In intergalactic space, only the brightness left by the Big Bang heats the substance significantly.
Visible (left) and infrared (right) view of the dusty Bok sphere, Barnard 68. It has a temperature below 20 K, so it remains invisible in both the visible and near-infrared, but it is still quite hot compared to temperatures on the cosmic microwave background.
At 2725 K above absolute zero, only places that are actively cooling are cooler.
A color-coded image of the Boomerang Nebula, taken by the Hubble Space Telescope. The gas emitted from this star expanded at an incredible rate, causing adiabatic cooling. There are places inside that are colder than even the afterglow from the Big Bang itself.
(Credit: NASA, ESA and Hubble Heritage Team (STScI / AURA))
However, many mechanisms heat the fabric of the universe.
The picture shows the central area of the Tarantula Nebula in the large Magellanic cloud. The young and dense star cluster R136 is visible at the bottom right of the image. Tidal forces exerted on the great Magellanic cloud at the Milky Way trigger a wave of star formation there, resulting in hundreds of thousands of new stars. The injection of energy into the universe by the formation of stars is one of the most important sources of warming matter in galactic environments.
(Credit: NASA, ESA and P. Crowther (University of Sheffield))
Stars, for example, produce radiation that hits nearby gas and dust.
This distant infrared image of Messier 16, the Eagle Nebula, shows a series of neutral atoms heated between 10 K (red) and 40 K (blue) by the stars already formed in them. Below the center of the image, the famous Columns of Creation are visible far infrared, a unique image of this object thanks to the still unsurpassed capabilities of ESA’s Herschel Observatory.
(Credit: ESA / Herschel / PACS / SPIRE / Hill, Motte, HOBYS Key Program Consortium)
Heated to tens of thousands of degrees above absolute zero, radiating in the far infrared.
The famous Colonies of Creation in the Eagle Nebula is a place where new stars are formed in a race against evaporating gas. In the visible light, on the left, the new stars are largely hidden, while the infrared light allows us to see through the dust to the stars and form protostars in them. Even cooler gas will radiate at longer wavelengths.
(Credit: NASA, ESA and Hubble Heritage Team (STScI / AURA))
Closer to a newly formed star, the radiation creates protoplanetary structures.
A sample of 20 protoplanetary disks around young stars and infant stars, measured by the High Angular Resolution Disk Substructures Project: DSHARP. Observations like these have taught us that protoplanetary disks are mostly formed in a single plane and tend to support the basic growth scenario for planet formation. Disc structures are visible in both infrared and millimeter / submillimeter wavelengths.
(Credit: SM Andrews et al., ApJL, 2018)
Heated to hundreds of degrees, these protoplanetary disks radiate into the infrared.
The star-forming region Sh 2-106 shows an interesting range of phenomena, including illuminated gas, a bright central star that provides this illumination, and blue reflections from gas that have not yet been expelled. The different stars in this region probably come from a combination of stars from different past and generational histories, but none of them are pristine: they all contain significant amounts of heavy elements.
(Credit: ESA / Hubble and NASA)
However, high-energy events can have dramatic astronomical consequences.
This image shows the open star cluster NGC 290, seen by Hubble. These stars, shown here, show a variety of colors because they have different temperatures, so warmer stars emit more blue light than red, while cooler stars emit more red than blue light. The different colors can only be detected by imaging stars at several different wavelengths, but it is the blown, warmest and clearest stars that primarily cause ambient matter to heat up and ionize. .
(Credit: ESA and NASA; Credit: Davide de Martin (ESA / Hubble) and Edward W. Olszewski (University of Arizona)
The warmest, most massive young stars shine brightly in ultraviolet light.
Most galaxies contain only a few star-forming regions: where the gas collapses, new stars form, and ionized hydrogen is found in a bubble that surrounds this region. In a stellar galaxy, almost the entire galaxy itself is a star-forming region, with M82, the cigar galaxy, being the closest with these properties. Radiation from hot young stars ionizes a range of atomic and molecular gases, creating emission signatures that can be visually revealed with the right astronomical filters.
(Credits: NASA, ESA and Hubble Heritage Team (STScI / AURA); Awards: J. Gallagher (University of Wisconsin), M. Mountain (STScI) and P. Puxley (National Science Foundation))
The radiation heats the gas to thousands of degrees and ionizes many atoms and molecules.
When the central star of a dying star system is heated to temperatures of about 30,000 K, it becomes hot enough to ionize previously expelled material, creating a veritable planetary nebula in the case of a sun-like star. Here, NGC 7027 has just crossed this threshold and continues to grow rapidly. With only ~ 0.1-0.2 light-years in diameter, it is one of the smallest and youngest known planetary nebulae.
(Credit: NASA, ESA and J. Kastner (RIT))
When electrons lower their energy levels, they emit a variety of emission signatures.
The Great Magellanic Cloud is home to the nearest supernova from the last century. The pink areas here are not man-made, but are signs of ionized hydrogen and active star formation, probably caused by gravitational interactions and tidal forces. The pink areas appear specifically when electrons fall back into ionized hydrogen nuclei and go from energy level n = 3 to energy level n = 2, producing photons of exactly 656.3 nm.
(Credit: Jesus Pelaez Aguado)
At a few thousand degrees, hydrogen ionizes, making mists pink with emission lines.
Around a variety of stellar and dying stars, double-ionized oxygen atoms produce a characteristic green glow, while electrons cascade through different energy levels when heated to temperatures above ~ 50,000 K. Here, the planetary nebula IC 1295 shines clear.
Above ~ 50,000 K, around dying stars, double-ionized oxygen glows eerily green.
This image from NASA’s Chandra X-ray Observatory shows the location of various elements in the Cassiopeia A supernova remnant, including silicon (red), sulfur (yellow), calcium (green) and iron (purple). . Each of these elements produces X-rays in narrow energy ranges, making it possible to make maps of their location.
(Credit: NASA / CXC / SAO)
The colliding galaxies further heat the gas and generate X-rays.
X-ray (pink) and general substance (blue) maps of several colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. X-rays come in two variants, soft (lower energy) and hard (higher energy), where colliding galaxies can create temperatures of over several hundred thousand degrees.
(Credit: NASA, ESA, D. Harvey (Ecole Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (University of Durham, UK), T. Kitching (University College London, UK) and A Taylor and E. Tittley (University of Edinburgh, UK)
But bright neutron stars and black holes can shape entire galaxies.
The radio features shown here in orange highlight the giant Alcyoneus radio galaxy along with the central black hole, its jets, and the patches at each end. This feature is the largest known in the universe for a single galaxy and makes Alcyoneus the largest known galaxy in the universe today. Although only radio and infrared functions are represented here, it also radiates in the high energy part of the spectrum.
(Credit: MSSL Oei et al., Astronomy and Astrophysics, 2022)
Produces gamma photons, the highest energy available, even the Large Hadron Collider cannot compete.
Fermi’s view of the gamma ray sky reveals emissions from our own galaxy, extragalactic objects, pulsars and, as highlighted here, also supernova remnants.
(Credit: NASA / DOE / Fermi LAT Collaboration)
Mostly Mute Monday tells an astronomical story in pictures, pictures and no more than 200 words. Talk less; smile more.