Solving the mystery of nebulae in astronomy

From a “faint smudge” to the most intricate objects in our galaxy, we’ve come an incredibly long way.

“From our home on the Earth, we look out into the distances … to imagine the sort of world into which we are born… But with increasing distance our knowledge fades, and fades rapidly, until at the last dim horizon we search among ghostly errors of observations for landmarks that are scarcely more substantial. The search will continue. The urge is older than history. It is not satisfied and it will not be suppressed.” –Edwin Hubble

From ancient times, humanity realized there’s more to the night sky than stars.

Located just outside the Big Dipper, the objects M81 and M82 appear nebulous, but are in fact galaxies located far outside the Milky Way, containing billions of stars apiece. Image credit: Markus Schopfer under a c.c.-by-2.5 license.

Stellar assemblies, like clusters and galaxies, are plentiful, but aren’t real nebulae.

*A section of the sky with many nebulous features around R Coronae Australis. Image credit: ESO.*

A true nebulae, is a diffuse, cloud-like object, resulting from gas within a galaxy.

*A dark nebula images in the nebula NGC 281, by the Hubble Space Telescope. Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA); Acknowledgment: P. McCullough (STScI).*

Dark nebulae — dusty, dense clouds of material — block incoming background light.

The dark cloud Barnard 68 blocks the background light from all the stars behind it. Data obtained with the 8.2-m VLT ANTU telescope and the multimode FORS1 instrument in March 1999. Image credit: ESO.

Many will form stars in the future, with their gas not having collapsed enough yet.

*The reflection nebula NGC 1999. Image credit: NASA/ESA and the Hubble Heritage Team (STScI).*

Some nebulae appear blue, reflecting the light from bright, newly-formed nearby stars.

*Reflection nebula IC 2631, as imaged by the MPG/ESO 2.2-m telescope. Image credit: ESO.*

These “reflection nebulae” are often dominated by just a single luminous, young star.

*The central region of the emission nebula N44 in the Large Magellanic Cloud. Image credit: ESO.*

Other nebulae appear red, due to ionized electrons falling back down onto ionized hydrogen atoms.

The great Orion Nebula is a fantastic example of an emission nebula, as evidenced by its red hues and its characteristic emission at 656.3 nanometers. Image credit: NASA, ESA, M. Robberto (Space Telescope Science Institute/ESA) and the Hubble Space Telescope Orion Treasury Project Team.

These “emission nebulae” surround star-forming regions, exhibiting spectral lines peaking at 656 nanometers.

*Herbig Haro object HH47, observed with the Hubble Space Telescope/WFPC2. Image credit: J. Morse/STScI, and NASA.*

Other causes of nebulae are gaseous outflows from massive stars, like Herbig-Haro objects.

The supernova remnant SNR 0509 displays a delicate shell in visible light; X-rays and other wavelengths show a much more complex structure. Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA). Acknowledgement: J. Hughes (Rutgers University).

Supernovae create their own nebulae from the remnants of their catastrophic explosions.

The Cat’s Eye Nebula is a formerly Sun-like star that has ejected its outer layers while the central remnant contracts down to a hot white dwarf. Image credit: NASA, ESA, HEIC, and The Hubble Heritage Team (STScI/AURA).

Finally, Sun-like stars run out of fuel, expelling their outer layers into planetary nebulae.

*The Egg Nebula, as imaged here by Hubble, is a preplanetary nebula, as its outer layers have not yet been heated to sufficient temperatures by the central, contracting star. Image credit: NASA.*

Previously, outflowing matter from these stars create pre-planetary nebulae.

*The Helix Nebula, shown above, has evolved into a full-blown planetary nebula. Image credit: NASA, ESA, and C.R. O’Dell (Vanderbilt University), for the Hubble Helix Team.*

It’s only when the central star reaches 30,000 K, ionizing the surrounding gas, that a true planetary nebula occurs.

Mostly Mute Monday tells the story of a single astronomical phenomenon or object in visuals, images and video in no more than 200 words.

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