Messier Monday: The Dumbbell Nebula, M27

The brightest “planetary nebula” is actually a Sun-like star in its final death throes!


“When he shall die,
Take him and cut him out in little stars,
And he will make the face of heaven so fine
That all the world will be in love with night
And pay no worship to the garish sun.” -
William Shakespeare

When you think of the stars in the night sky, they may seem eternal, but you know better. In fact, the vast majority of the brightest ones we can see are either very bright, young blue giants that are destined to be short lived, or old, red giants that have burned through the majority of their fuel. In both cases, these stars are nearing the end of their lives. But did you know that — just beyond the reach of your naked eye’s capabilities — lie a number of recently deceased red giants that were bright, shining stars just a few hundred thousand years ago?

Image credit: Rich Richins’ 2009 Messier Marathon, retrieved via http://starizona.com/acb/.

To the very first humans on Earth, these stellar remnants would have been visible as true, simple stars: points of light in the sky. There are four objects like this in the Messier catalogue, the first large-scale compilation of deep sky objects easily visible with simple equipment (a small telescope or binoculars) from Earth. Today’s object — Messier 27, the Dumbbell Nebula — was not only the first one discovered, it’s also the brightest one from our vantage point. As the solstice has recently passed us, it will be a spectacular object all summer long! Here’s how to find it.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

After the Sun goes down, stars begin to come out en masse in the night sky. The brightest ones appear first, and among them in the East are the three stars making up the prominent Summer Triangle: the trio of Vega, Altair and Deneb. (They are the fifth, twelveth, and nineteenth brightest stars in the entire sky, respectively.) Vega is easily identifiable as the brightest of the three, while Deneb and Altair begin the night a little closer to the horizon.

If you start from Deneb and navigate towards the opposite side of this triangle, you’ll see a pattern of six stars that are sometimes known as the Northern Cross.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

But it’s what you’ll see if you begin at Altair and navigate back towards Deneb that interests you tonight if you’re hunting for Messier 27! About a quarter of the way back to Deneb, you’ll find the prominent orange giant γ Sagittae, a star much younger than our Sun is. This star was born some 750 million years ago as a massive, blue B-class star. It’s run out of the hydrogen fuel that was initially in its core and has since expanded to become a red giant, burning helium in its central region instead.

Image credit: me, using the free software Stellarium, via http://stellarium.org/.

At some point in the future, it — and all Sun-like stars (stars between about 40% and 400% of our Sun’s mass) — will run out of helium as well, and when it does, it might begin to look an awful lot like Messier 27. If you continue the line connecting Altair to γ Sagittae by another 2°, you’ll come to a barely visible naked-eye star: 14 Vulpeculae. The faint, extended “cloud-like” object you’ll see right next to it through binoculars or a small telescope is the Dumbbell Nebula you’re looking for.

Image credit: NASA, ESA, and the Digitized Sky Survey 2. Acknowledgment: Davide De Martin (ESA/Hubble).

This object — the first of its class — was discovered by Charles Messier himself back in 1764, who described it thus:

Nebula without star, discovered in Vulpecula, between the two forepaws, & very near the star 14 of that constellation, of 5th magnitude according to Flamsteed; one can see it well with an ordinary telescope […] it appears of oval shape, & it contains no star.

Even through a good-sized telescope, the human eye will only perceive this as white, although the two-lobed structure will be clear. But color astrophotography brings out some spectacular detail here.

Image credit: Alistair Symon of Woodlands Observatory, via http://www.woodlandsobservatory.com/M27/M27170606.htm.

You’ll notice that there appears to be a star at the center of this nebula, and this is no mere coincidence or chance alignment: this once-giant star has collapsed down to a dwarf, but a tremendously hot one at about 85,000 K! (About 14 times as hot as the surface of our Sun, for comparison.)

The different colors are indicative of different elements in different ionization states, with red being a telltale sign of ionized hydrogen. But if you focus in on the narrow-band colors that other elements make, you’ll find something more than simple hydrogen.

Image credit: European Southern Observatory, via http://www.eso.org/public/images/eso9846a/.

The bright green glow you see is highlighting oxygen atoms in their rare, doubly-ionized state (where it’s missing two electrons). How did we arrive at this configuration, with two lobes of hydrogen gas on the outskirts, a sea of very hot oxygen gas towards the center, and an ultra-hot dwarf star at the center?

Image credit: Val Ricks of the Huntsville Amateur Astronomy Society, via http://www.huntsvilleastronomy.org/?page_id=50.

You see, this nebula is at most 48,000 years old, with many estimates placing it closer to the 10,000 year range! Prior to that, it was a giant star, burning through helium fuel in its core and hydrogen in its outer layers. When the core ran out of helium fuel, it contracted and heated up, and the elevation in radiation pressure first blew off the star’s outer, hydrogen-rich layers.

But it didn’t simply blow those layers off spherically, as our Sun most probably will. You see, Messier 27 is part of a binary star system, and that second star — the one that’s still burning through its fuel — helps eject that blown-off mass in the dual-lobed trajectory we see. In fact, some of the mass of the original star is flung even farther than typical images of this nebula show!

Image credit: Matthew T. Russell, via http://apod.nasa.gov/apod/ap100826.html.

In addition to the hydrogen, the relatively rapid contraction of this star causes it to heat up, and these increased temperatures trigger bursts of nuclear fusion that can expel portions of the inner layers of the star as well. Among other elements like sulphur and nitrogen, that’s where the oxygen comes from!

The inner core of this star will eventually contract down to a white dwarf: a very hot star that shines because of the energy of its gravitational contraction! This core is about the mass of the Sun, but it’s contracted down to only about the physical size of the Earth. This stores a tremendous amount of heat and energy in the dwarf star, but because it’s so small, it can only radiate this energy away very slowly. It will take somewhere around 10^14 years — or about 10,000 times the age of the present Universe — for this object to cool enough to stop emitting visible light.

Image credit: NASA/JPL-Caltech/J. Hora (Harvard-Smithsonian CfA), of the Spitzer Space Telescope’s Infrared Array Camera.

In the meantime, the nebula is expanding, as the gas that’s “gently” blown off from this until-recently-a-normal-star returns to the interstellar medium, where it will help form future generations of stars, planets, complex chemicals, and possibly life. When we’re looking at a planetary nebula, we’re watching part of the cosmic life cycle of death and rebirth.

And if you take a close look with, say, the Hubble Space Telescope, you’ll find that there are many “knots” of gas, rich in heavy elements, that form in this (and all) planetary nebulae!

Image credit: NASA and The Hubble Heritage Team (STScI/AURA); Acknowledgment: C.R. O’Dell (Vanderbilt University), via http://hubblesite.org/newscenter/archive/releases/2003/06/image/a/.

So enjoy this treasure of the night skies — the brightest planetary nebula — and think of all the hundreds of billions (if not trillions) of stars that already went through this in our galaxy alone to help create the solar systems, planets, and life that we have right now.

And that’s your connection to the cosmos on this Messier Monday! Take a look back at all our previous Messier Mondays here:

Come back tomorrow for more wonders of the Universe, and don’t forget to join us next week for another deep sky wonder on another Messier Monday!