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Decoding Nebulae

We can agree that nebulae are some of the most majestic-looking objects in the universe. But what are they exactly? Nebulae are giant clouds of gas and dust in space. They’re commonly associated with two parts of the life cycle of stars: First, they can be nurseries forming new baby stars. Second, expanding clouds of gas and dust can mark where stars have died.

A rotating gif of examples of a stellar nursery, NGC 346, and a planetary nebula, the Helix Nebula. The stellar nursery is a blue wispy cloud taking up nearly the entire scene and bright stars can be see dotted throughout the image, with bright ones near the center of the cloud. The Helix nebula looks like a giant eye with a blue center and pale yellow light fading into orange to form the “white” of the eye. redit: Helix Nebula image: NASA, NOAO, ESA, the Hubble Helix Nebula Team, M. Meixner (STScI), and T.A. Rector (NRAO); NGC 346: NASA, ESA and A. Nota (STScI/ESA)
This GIF rotates between two images: one of a planetary nebula called the Helix Nebula and one of a stellar nursery called NGC 346. The Helix Nebula shows a fine web of filamentary "bicycle-spoke" features embedded in a colorful red and blue ring of gas. This type of nebula is the glowing gas around a dying, Sun-like star. NGC 346 has a population of infant stars still forming from gravitationally collapsing gas clouds. They have not yet ignited their hydrogen fuel to sustain nuclear fusion. The smallest of these infant stars is only half the mass of our Sun.

Not all nebulae are alike, and their different appearances tell us what’s happening around them. Since not all nebulae emit light of their own, there are different ways that the clouds of gas and dust reveal themselves. Some nebulae scatter the light of stars hiding in or near them. These are called reflection nebulae and are a bit like seeing a streetlamp illuminate the fog around it.

A gray haze fills most of the frame with a large black region in the shape of a game pawn tilted about 45-degrees to the left. Credit: NASA and The Hubble Heritage Team (STScI)
A dusty bright nebula contrasts dramatically with a dusty dark nebula in this Hubble Space Telescope image. The nebula, cataloged as NGC 1999, is a reflection nebula, which shines by reflecting light from a nearby star. Unlike emission nebulae, whose reddish glow comes from excited atoms of gas, reflection nebulae have a bluish cast as their interstellar dust grains preferentially reflect blue starlight.
NASA and The Hubble Heritage Team (STScI)

In another type, called emission nebulae, stars heat up the clouds of gas, whose chemicals respond by glowing in different colors. Think of it like a neon sign hanging in a shop window!

Near the center of the image shines a bright star with four spikes. It forms a vertex of a triangle of wispy blue and blue-white light that forms the emission nebula taking up about a third of the image. To the right and above the bright star, the image quickly fades from pale red to black, which is where a dense cloud of dust blocks our view of the nebula. Credit: ESA/Hubble, R. Sahai
Emission nebulae are bright, diffuse clouds of ionized gas that emit their own light. In NGC 2313, pictured here, the bright star V565 (center of the image) highlights a silvery veil of gas and dust, while the right half of this image is obscured by a dense cloud of dust.
ESA/Hubble, R. Sahai

Finally there are nebulae with dust so thick that we’re unable to see the visible light from young stars shine through it. These are called dark nebulae.

Against a starry background, this nebula looks like a flowing dress with a reddish-white bodice and flowing faint blue skirt. In the foreground are a few bright stars, each having four rainbow spikes. Credit: NASA, ESA, and R. Sahai (Jet Propulsion Laboratory); Processing: Gladys Kober (NASA/Catholic University of America)
Caldwell 99 is a dark nebula — a dense cloud of interstellar dust that completely blocks out visible wavelengths of light from objects behind it. The object at the center of the image is a (much smaller) protoplanetary nebula, which represents a late stage in the life of a star that has ejected a shell of hydrogen gas and is quickly heating up.
NASA, ESA, and R. Sahai. (Processing: Gladys Kober)

Our missions help us see nebulae and identify the different elements that oftentimes light them up.

The Hubble Space Telescope is able to observe the cosmos in multiple wavelengths of light, ranging from ultraviolet, visible, and near-infrared. Hubble peered at the iconic Eagle Nebula in visible and infrared light, revealing these grand spires of dust and countless stars within and around them.

This pair of images of the Eagle Nebula in visible and near-infrared light were taken by the Hubble Space Telescope. In optical light a dark cloud with three “fingers” poking across the image from the left on a background awash in blue and orange cloudy light. The edges of the dark regions glow with white and light-blue light. In infrared light the orange and blue clouds fade into a dark background, revealing a densely populated star field. Between the fingers and at their tips, there is still a glow of faint blue light. Credit: NASA, ESA and the Hubble Heritage Team (STScI/AURA)
These tendrils of cosmic dust and gas sit at the heart of M16, or the Eagle Nebula. The aptly named Pillars of Creation, featured in these stunning Hubble images, are part of an active star-forming region within the nebula and hide newborn stars in their wispy columns. In the optical light image, blue represents oxygen, red is sulfur, and green signals both nitrogen and hydrogen. The second, darker image shows Hubble’s infrared view, piercing through the obscuring gas and dust of M16’s Pillars of Creation.
NASA, ESA, and G. Bacon (STScI)

The Chandra X-ray Observatory studies the universe in X-ray light! The spacecraft is helping scientists see features within nebulae that might otherwise be hidden by gas and dust when viewed in longer wavelengths like visible and infrared light. In the Crab Nebula, Chandra sees high-energy X-rays from a pulsar (a type of rapidly spinning neutron star, which is the crushed, city-sized core of a star that exploded as a supernova).

The X-ray emission from the Crab nebula is shown in shades of blue. At the center is a bright ball surrounded by an oval of light whose long axis is tilted just to the left. Rising up and to the right from that oval is a cloud of material that looks like a witch’s hat with a broad brim and dented point. From the central ball there is also a bent stream of material to the left and downward. Credit: NASA/CXC/SAO
The Crab Nebula, imaged here by the Chandra X-ray Observatory, is powered by a quickly spinning, highly magnetized neutron star called a pulsar, which was formed when a massive star ran out of its nuclear fuel and collapsed.

The James Webb Space Telescope primarily observes the infrared universe. With Webb, scientists are peering deep into clouds of dust and gas to study how stars and planetary systems form.

An undulating, translucent star-forming region in the Carina Nebula is shown in this Webb image, hued in ambers and blues; foreground stars with diffraction spikes can be seen, as can a speckling of background points of light through the cloudy nebula. Credit: NASA's Goddard Space Flight Center
The seemingly three-dimensional “Cosmic Cliffs” showcase the James Webb Space Telescope’s capabilities to peer through obscuring dust and shed new light on how stars form. Webb reveals emerging stellar nurseries and individual stars that are completely hidden in visible-light pictures. This landscape of “mountains” and “valleys” is actually the edge of a nearby stellar nursery called NGC 3324 at the northwest corner of the Carina Nebula.

The Spitzer Space Telescope studied the cosmos for over 16 years before retiring in 2020. With the help of its detectors, Spitzer revealed unknown materials hiding in nebulae — like oddly-shaped molecules and soot-like materials, which were found in the California Nebula.

This image shows two overlapping, but offset, images of the California Nebula. A central square where they overlap shows a black stary background with a cloudy gray nebula swoosh taking up about a third of the image. The image continues to the left of the square in blue hues, representing the image from one of the Spitzer detectors. The other image continues to the right in red hues, from another detector. Credit: NASA/JPL-Caltech
This series of infrared images taken by NASA's Spitzer Space Telescope on Jan. 25, 2020, shows part of the California Nebula, which is located about 1,000 light-years from Earth.

Studying nebulae helps scientists understand the life cycle of stars. Did you know our Sun got its start in a stellar nursery? Over 4.5 billion years ago, some gas and dust in a nebula clumped together due to gravity, and a baby Sun was born. The process to form a baby star itself can take a million years or more!

This visualization provides a three-dimensional perspective on Hubble's image of the nebula Gum 29 with the star cluster Westerlund 2 at its core. The flight traverses the foreground stars and approaches the lower left rim of the nebula Gum 29. Passing through the wispy darker clouds on the near side, the journey reveals bright gas illuminated by the intense radiation of the newly formed stars of cluster Westerlund 2. Within the nebula, several pillars of dark, dense gas are being shaped by the energetic light and strong stellar winds from the brilliant cluster of thousands of stars. Credit: NASA, ESA, G. Bacon, L. Frattare, Z. Levay, and F. Summers (Viz3D Team, STScI), and J. Anderson (STScI)
A visualization flying into the nebula Gum 29 and the star cluster Westerlund 2 at its core.
NASAESA, Greg Bacon, Lisa Frattare, Zolt Levay, and Frank Summers (Viz3D Team, STScI), and Jay Anderson (STScI) (Acknowledgment: The Hubble Heritage Team (STScI/AURA), A. Nota (ESA/STScI), the Westerlund 2 Science Team, and ESO)

After billions more years, our Sun will eventually puff into a huge red giant star before leaving behind a beautiful planetary nebula (so-called because astronomers looking through early telescopes thought they resembled planets), along with a small, dense object called a white dwarf that will cool down very slowly. In fact, we don’t think the universe is old enough yet for any white dwarfs to have cooled down completely.

Since the Sun will live so much longer than us, scientists can’t observe its whole life cycle directly … but they can study tons of other stars and nebulae at different phases of their lives and draw conclusions about where our Sun came from and where it’s headed. While studying nebulae, we’re seeing the past, present, and future of our Sun and trillions of others like it in the cosmos.

A circular view of the sky captured by NASA’s TESS mission unfolds like a paper fan around a central point in this animated GIF. Each segment looks like a film strip studded with stars. Credit: NASA/MIT/TESS and Ethan Kruse (USRA)
NASA’s Transiting Exoplanet Survey Satellite (TESS) spent a year imaging the southern sky full of all kinds of stars during its search for worlds beyond our solar system.
NASA/MIT/TESS and Ethan Kruse (USRA)