The space between stars is dotted with twisting towers studded with stars, unblinking eyes, ethereal ribbons, and floating bubbles. These fantastical shapes, some of the universe’s most visually stunning constructions, are nebulae, clouds of gas and dust that can be the birthplace of stars, the scene of their demise – and sometimes both.

Nebulae are made up of gas – primarily hydrogen and helium – and fine cosmic dust. These clouds are part of the interstellar medium of extremely low-density gas and dust that exists between stars in the void of space, material so dispersed through the vastness of the cosmos that it can have a density as low as 0.1 atoms per cubic centimeter. In contrast, a cubic centimeter of the air we breathe on Earth would contain about 10 million trillion molecules.

Nebulae happen in places where the interstellar medium has become dense enough to form clouds. That could be because gravity has pulled the gas and dust together, or because stars have died and spewed their contents into the cosmos. Several types of nebulae exist, and each individual nebula is a unique showcase of the phenomena and processes happening within. They often live side by side, with smaller nebulae populating larger clouds of gas and dust to form giant complexes of star birth and death.

Emission Nebulae

Emission nebulae are so named because they emit their own light. This type of nebula forms when the intense radiation of stars within or near the nebula energizes the gas. A star's ultraviolet radiation floods the gas with so much energy that it strips electrons from the nebula's hydrogen atoms, a process called ionization. As the energized electrons revert from their higher-energy state to a lower-energy state by recombining with atoms, they emit energy in the form of light, causing the nebula's gas to glow.

Bright cloud of pink, white, orange, and yellow with dark dust lanes.
The Orion Nebula is a picture book of star formation, from the massive, young stars shaping the nebula to the pillars of dense gas that may be the homes of budding stars. The Trapezium resides in the bright central region. Ultraviolet light unleashed by these stars carves a cavity in the emission nebula and disrupts the growth of hundreds of smaller stars.
Credits: NASA, ESA, M. Robberto (STScI/ESA), and the Hubble Space Telescope Orion Treasury Project Team

A famous example of an emission nebula is the Orion Nebula, a huge, star-forming nebula in the constellation Orion. The Orion Nebula is home to a star cluster defined by four massive stars known as the Trapezium. These stars are only a few hundred thousand years old, about 15-30 times the mass of our Sun, and so hot and bright that they're responsible for illuminating the entire Orion Nebula. But thousands of additional, mostly young stars are embedded in the nebula. The most massive are 50 to 100 times the mass of our Sun.

The radiation and solar winds of stars within emission nebulae carve and sculpt the nebula's gas, creating caverns and pillars but also creating pressures on the gas clouds that can give rise to more starbirth.

Reflection Nebulae

Reflection nebulae reflect the light from nearby stars. The stars that illuminate them aren’t powerful enough to ionize the nebula’s gas, as with emission nebulae, but their light scatters through the gas and dust causing it to glow – like a flashlight beam shining on mist in the dark.

Because of the way light scatters when it hits the fine dust of the interstellar medium, these reflection nebulae are often bluish in color.

A reflection nebula called NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth. The nebula is illuminated by a bright, recently formed star called V380 Orionis, and the gas and dust of the nebula is material left over from that star’s formation. A second well-known reflection nebula is illuminated by the Pleiades star cluster. Most nebulae around star clusters consist of material that the stars formed from. But the Pleiades shines on an independent cloud of gas and dust, drifting through the cluster at about 6.8 miles/second (11 km/s).

Faint, wispy clouds glow from the light of a central, pale star. A hole in the clouds, roughly in the shape of a keyhole, shows black space.
The star V830 Orionis shines on the cloud of material left over from its formation, here seen as the NGC 1999 reflection nebula.
Credits: NASA and The Hubble Heritage Team (STScI)

Planetary Nebulae

When astronomers looked at the sky through early telescopes, they found many indistinct, cloudy forms. They called such objects "nebulae," Latin for clouds. Some of the fuzzy objects resembled planets, and these earned the name "planetary nebulae."

Black background. At image center is a colorful ring of gas and dust. The ring is a deep red along the outside. Moving inward toward the center of the ring, the colors change from red, to orange, to yellow, to pinkish-white, to a light blue sphere in the center.
The Helix Nebula is an example of a planetary nebula. Though it looks like a bubble or eye from Earth's point of view, the Helix is actually a trillion-mile-long tunnel of glowing gases. In its center lies a white dwarf star.
Credits: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO)

Today these nebulae keep the name, but we know they have nothing to do with planets. Planetary nebulae form during the death of low-mass to medium-mass stars. When such stars die, they expel their outer layers into space. These expanding shells of gas form a huge variety of unique shapes – rings, hourglasses, rectangles, and more – that show the complexity of stellar death. Astronomers are still studying how these intricate shapes form at the end of a star's life.

As the star casts off its outer layers, it leaves behind its core, which becomes a white dwarf star. White dwarf stars are objects with the approximate mass of the Sun but the size of the Earth, making them one of the densest forms of matter in the universe after black holes and neutron stars. The white dwarf star's ultraviolet radiation ionizes the gas of the planetary nebula and causes it to glow, just as stars do in emission nebulae. Our Sun is expected to form a planetary nebula at the end of its life.

Supernova Remnants

Not all stars die gently, exhaling their outer layers into space. Some explode in a supernova, flinging their contents into space at anywhere from 9,000 to 25,000 miles (15,000 to 40,000 kilometers) per second.

When a star has a lot of mass – at least five times that of our Sun – or is part of a binary system in which a white dwarf star can gravitationally pull mass from a companion star, it can explode with the brightness of 10 billion Suns. Supernova remnants consist of material from the exploded star and any interstellar material it sweeps up in its path.

The new debris from the explosion and material ejected by the star earlier in its life collide, heating up in the shock until it glows with x-rays. Supernova remnants’ glow can also be powered by the stellar wind of a pulsar – a rapidly spinning neutron star created from the core of the exploded star. The pulsar emits electrons that interact with the magnetic field it produces, a process called synchrotron radiation, and emits X-rays, visible light and radio waves.

The Crab Nebula is an example of a supernova remnant. The explosion that created it in the year 1054 was so bright that for weeks it could be seen even in the daytime sky, and it was recorded by astronomers across the world. The material from the star is still rushing outward at around 3 million mph (4.8 million kph).

Bright green, orange, and yellow tendrils intertwined within this egg shaped nebula.
The Crab Nebula is an expanding remnant of a star's supernova explosion. Japanese and Chinese astronomers recorded this violent event nearly 1,000 years ago in 1054 AD, as likely did the Native Americans. The glowing relic has been expanding since the star exploded, and it is now approximately 11 light-years in width. This Hubble mosaic is one of the largest images ever taken of a supernova remnant by the space telescope. It is also the highest resolution image ever made of the entire Crab Nebula, which is located 6,500 light-years away. The composite was assembled from 24 individual exposures taken with Hubble's Wide Field and Planetary Camera 2 in October 1999, January 2000, and December 2000. The orange filaments are the tattered remains of the star and consist mostly of hydrogen. The rapidly spinning neutron star embedded in the center of the nebula is the dynamo powering the nebula's eerie interior bluish glow. The blue light comes from electrons whirling at nearly the speed of light around magnetic field lines from the neutron star, which is the crushed, ultra-dense core of the exploded star. Like a lighthouse, the neutron star produces twin beams of radiation. From Earth, it appears to pulse 30 times a second due to the neutron star's rotation sweeping the beams across our line of sight. It has the mass equivalent to the Sun crammed into a rapidly spinning ball of neutrons 12 miles across. The nebula derived its name from its appearance in an 1844 drawing made by Irish astronomer Lord Rosse, who used a 36-inch telescope. When viewed by Hubble, as well as by large, ground-based telescopes, the Crab Nebula takes on a more detailed appearance that yields clues into the spectacular demise of the star. It has been found that the knots lie relatively close to the source of the ionizing radiation, which may lead to higher gas temperatures of the knots than expected. This limits our understanding of the structure of the nebula and what role magnetic fields may play as the material expands outward and eventually combines with other material to form new stars. Hubble has been used to determine several northwest-southeast (upper right to lower left) asymmetries in the nebula's filaments, as well as the development of long "fingers" of gas and dust. This has been attributed to the sideways motion of the neutron star in the northwest (upper right) direction. Hubble observations of the Crab Nebula along with data from other observatories have been used to investigate the expansion and magnetic fields of the nebula remnant from the explosion. For more information please visit: hubblesite.org/contents/news-releases/2005/news-2005-37.html
NASA, ESA, and the Hubble Heritage Team (STScI/AURA)

Absorption Nebulae

Absorption nebulae or dark nebulae are clouds of gas and dust that don’t emit or reflect light, but block light coming from behind them. These nebulae tend to contain large amounts of dust, which allows them to absorb visible light from stars or nebulae beyond them. Astronomer William Herschel, discussing these seemingly empty spots in the late 1700s, called them “a hole in the sky.”

Included among absorption nebulae are objects like Bok globules, small, cold clouds of gas and dense cosmic dust. Some Bok globules have been found to have warm cores, which would be caused by star formation inside, and further observation has indicated the presence of multiple stars of varying ages, suggesting a slow, ongoing star formation process.

Glowing reddish gas is speckled with bright-white stars. In the foreground is a dark, vertically extended cloud of gas and dust that blocks the light of objects behind it. The largest part of the dark cloud (Bok Globule) extends from image center to bottom. Tenuous clouds extend vertically above the main globule toward the image top.
These opaque, dark knots of gas and dust called "Bok globules" are absorbing light in the center of the nearby emission nebula and star-forming region, NGC 281. Bok globules may form stars, or may eventually dissipate.
Credits: NASA, ESA, and The Hubble Heritage Team (STScI); Acknowledgment: P. McCullough (STScI)

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