Exploring the Carina Nebula

Image of the Week for Light and Shadow in the Carina Nebula
Previously unseen details of a mysterious, complex structure within the Carina Nebula (NGC 3372) are revealed by this image obtained by NASA's Hubble Space Telescope. NASA, Hubble Heritage Team and N Walborn (STScI), R Barba (La Plata Observatory) and A Caulet

When astronomers want to look at all the stages of star birth and star death in the Milky Way galaxy, they often turn their gaze to the mighty Carina Nebula, in the heart of the constellation Carina. It's often referred to as the Keyhole Nebula due to its keyhole-shaped central region. By all standards, this emission nebula (so-called because it emits light) is one of the largest that can be observed from Earth, dwarfing the Orion Nebula in the constellation Orion. This vast region of molecular gas is not well-known to observers in the northern hemisphere since it's a southern skies object. It lies against the backdrop of our galaxy and almost seems to blend in with that band of light that stretches across the sky.

Since its discovery, this giant cloud of gas and dust has fascinated astronomers. It provides them a one-stop location to study the processes that form, shape, and ultimately destroy stars in our galaxy. 

Behold the Vast Carina Nebula

Image of the Week for Dark Clouds of the Carina Nebula
The Carina Nebula (in the Southern Hemisphere sky) is home to many massive stars, including HD 93250, hidden among its clouds. NASA, ESA, N. Smith (U. California, Berkeley) et al., and Hubble Heritage Team (STScI/AURA)

The Carina nebula is part of the Carina-Sagittarius arm of the Milky Way. Our galaxy is in the shape of a spiral, with a set of spiral arms arcing around a central core. Each set of arms has a specific name.

The distance to the Carina Nebula is somewhere between 6,000 and 10,000 light-years away from us. It's very extensive, stretching across some 230 light-years of space and is quite a busy place. Within its boundaries are dark clouds where newborn stars are forming, clusters of hot young stars, old dying stars, and the remnants of stellar behemoths that have already blown up as supernovae. Its most famous object is the luminous blue variable star Eta Carinae.

The Carina Nebula was discovered by the astronomer Nicolas Louis de Lacaille in 1752. He first observed it from South Africa. Since that time, the expansive nebula has been studied intensely by both ground-based and space-based telescopes. Its regions of star birth and star death are tempting targets for Hubble Space Telescope, the Spitzer Space Telescope, the Chandra X-ray Observatory, and many others. 

Star Birth in the Carina Nebula

starforming region in carina nebula
Bok globules in the Carina Nebula are home to young stellar objecfts that are still fomring inside their clouds of gas and dust. The globules are shaped by hot winds from nearby stars. NASA-ESA/STScI

The process of star birth in the Carina Nebula follows the same path that it does in other clouds of gas and dust throughout the universe. The nebula's main ingredient — hydrogen gas — makes up the majority of the cold molecular clouds in the region. Hydrogen is the main building block of stars and originated in the Big Bang some 13.7 billion years ago. Threaded throughout the nebula are clouds of dust and other gases, such as oxygen and sulfur.

The nebula is studded with cold dark clouds of gas and dust called Bok globules. They are named for Dr. Bart Bok, the astronomer who first figured out what they were. These are where the first stirrings of star birth take place, hidden from view. This image shows three of these islands of gas and dust in the heart of the Carina Nebula. The process of star birth begins inside these clouds as gravity pulls material into the center. As more gas and dust clump together, temperatures rise and a young stellar object (YSO) is born. After tens of thousands of years, the protostar at the center is hot enough to begin fusing hydrogen in its core and it begins to shine. The radiation from the newborn star eats away at the birth cloud, eventually destroying it completely. Ultraviolet light from nearby stars also sculpts the star birth nurseries. The process is called photodissociation, and it is a by-product of star birth.

Depending on how much mass there is in the cloud, the stars born inside it can be around the mass of the Sun, or much, much larger. The Carina Nebula has many very massive stars, which burn very hot and bright and live short lives of a few millions of years. Stars like the Sun, which is more of a yellow dwarf, can live to be billions of years old. The Carina Nebula has a mix of stars, all born in batches and scattered through space.

Mystic Mountain in the Carina Nebula

Mystic Mountain in the Carina Nebula
A star-forming region called "Mystic Mountain" in the Carina Nebula. Its many peaks and "fingers" hide newly forming stars. NASA/ESA/STScI

As stars sculpt the birth clouds of gas and dust, they create amazingly beautiful shapes. In the Carina Nebula, there are several regions that have been carved away by the action of radiation from nearby stars.

One of them is Mystic Mountain, a pillar of star-forming material that stretches over three light-years of space. Various "peaks" in the mountain contain newly forming stars that are eating their way out while nearby stars shape the exterior. At the very tops of some of the peaks are jets of material streaming away from the baby stars hidden inside. In a few thousand years, this region will be home to a small open cluster of hot young stars within the larger confines of the Carina Nebula. There are many star clusters (associations of stars) in the nebula, which gives astronomers insight into the ways that stars are formed together in the galaxy. 

Carina's Star Clusters

trumpler 14 in Carina Nebula
Trumpler 14, part of the Carina Nebula, as seen by Hubble Space Telescope. This open cluster has many hot, young, massive stars. NASA/ESA/STScI

The massive star cluster called Trumpler 14 is one of the largest clusters in the Carina Nebula. It contains some of the most massive and hottest stars in the Milky Way. Trumpler 14 is an open star cluster that packs a huge number of luminous hot young stars packed into a region about six light-years across. It's part of a larger grouping of hot young stars called the Carina OB1 stellar association. An OB association is a collection of anywhere between 10 to 100 hot, young, massive stars that are still clustered together after their birth.

The Carina OB1 association contains seven clusters of stars, all born about the same time. It also has a massive and very hot star called HD 93129Aa. Astronomers estimate it to be 2.5 million times brighter than the Sun and it's one of the youngest of the massive hot stars in the cluster. Trumpler 14 itself is only about a half million years old. By contrast, the Pleiades star cluster in Taurus is about 115 million years old. The young stars in Trumpler 14 cluster sending furiously strong winds out through the nebula, which also help sculpt the clouds of gas and dust.

As the stars of Trumpler 14 age, they are consuming their nuclear fuel at a prodigious rate. When their hydrogen runs out, they'll begin to consume helium in their cores. Eventually, they'll run out of fuel and collapse on themselves. Eventually, these massive stellar monsters will explode in tremendous catastrophic outbursts called "supernova explosions." The shock waves from those explosions will ​send their elements out to space. That material will enrich future generations of stars to be formed in the Carina Nebula.

Interestingly, although many stars have already formed within the Trumpler 14 open cluster, there are still a few clouds of gas and dust remaining. One of them is the black globule at the center left. It may well be nurturing a few more stars that will eventually eat away their créche and shine forth in a few hundred thousand years.

Star Death in the Carina Nebula

ESO-Eta_Carinae-phot-17a-08-normal.jpg
A recent image of the star Eta Carinae taken at the European Southern Observatory. It shows double-lobed (bi-polar) structure and jets coming from the central star. The star has not yet blown up, but will soon. ESO

Not far from Trumpler 14 is the massive star cluster called Trumpler 16 — also part of the Carina OB1 association. Like its counterpart next door, this open cluster is chock full of stars that are living fast and will die young. One of those stars is the luminous blue variable called Eta Carinae.

This massive star (one of a binary pair) has been going through upheavals as a prelude to its death in a massive supernova explosion called a hypernova, sometime in the next 100,000 years. In the 1840s, it brightened up to become the second-brightest star in the sky. It then dimmed down for nearly a hundred years before beginning a slow brightening in the 1940s. Even now, it's a powerful star. It radiates five million times more energy than the Sun does, even as it prepares for its eventual destruction.

The second star of the pair is also very massive — about 30 times the mass of the Sun — but is hidden by a cloud of gas and dust ejected by its primary. That cloud is called "the Homunculus" because it seems to have a nearly humanoid shape. ​Its irregular appearance is something of a mystery; no one is quite sure why the explosive cloud around Eta Carinae and its companion has two lobes and is cinched in the middle.

When Eta Carinae blows its stack, it will become the brightest object in the sky. Over many weeks, it will slowly fade. Remnants of the original star (or both stars, if both explode) will rush out in shock waves through the nebula. Eventually, that material will become the building blocks of new generations of stars in the distant future.

How to Observe the Carina Nebula

Carina Nebula
A chart showing where the Carina Nebula is in the Southern Hemisphere skies. Carolyn Collins Petersen

Skygazers who venture to the southern reaches of the northern hemisphere and throughout the southern hemisphere can easily find the nebula in the heart of the constellation. It's very near the constellation Crux, also known as the Southern Cross. The Carina Nebula is a good naked-eye object ​and gets even better with a look through binoculars or a small telescope. Observers with good-sized telescopes can spend a lot of time exploring the Trumpler clusters, the Homunculus, Eta Carinae, and the Keyhole region at the heart of the nebula. The nebula is best viewed during the southern hemisphere summer and early autumn months (northern hemisphere winter and early spring).

Exploring the Life Cycle of Stars

For both amateur and professional observers, the Carina Nebula offers a chance to see regions similar to the one that birthed our own Sun and planets billions of years ago. Studying the starbirth regions in this nebula gives astronomers more insight into the process of starbirth and the ways that stars cluster together after they are born. In the distant future, observers will also watch as a star at the heart of the nebula explodes and dies, completing the cycle of star life.
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Petersen, Carolyn Collins. "Exploring the Carina Nebula." ThoughtCo, Aug. 28, 2017, thoughtco.com/carina-nebula-4149415. Petersen, Carolyn Collins. (2017, August 28). Exploring the Carina Nebula. Retrieved from https://www.thoughtco.com/carina-nebula-4149415 Petersen, Carolyn Collins. "Exploring the Carina Nebula." ThoughtCo. https://www.thoughtco.com/carina-nebula-4149415 (accessed December 16, 2017).