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Starts With A Bang

Messier Monday: A Cinco de Mayo Special, M4

Although we’re all under the same skies, those at Mexican latitudes will especially enjoy this one tonight!

“Rest is not idleness, and to lie sometimes on the grass under trees on a summer’s day, listening to the murmur of the water, or watching the clouds float across the sky, is by no means a waste of time.” –John Lubbock

Each Monday since late 2012, we’ve been taking an in-depth look at one of the 110 deep-sky wonders that make up the Messier Catalogue, the first accurate catalogue of star clusters, galaxies and nebulae visible with only a modest telescope ever created. Although I normally showcase objects visible from my latitude (45° N) in the early part of the night, for today — cinco de Mayo (May 5th) — I thought it only appropriate to showcase an object that rises early (at 8 PM) at Mexican latitudes of 19° N, even though observers as far north as I am will have to wait an extra three hours.

Image credit: Tenho Tuomi of Tuomi Observatory, via

You see, from any point on Earth, only half of the night sky is visible at any given time. After the Sun sets, different slices of the celestial skies become visible as the night goes on, providing skywatchers with a chance to glimpse a variety of stars, constellations and deep-sky objects. For today’s Messier Monday, even though the Moon will be out and illuminating the sky, it’s still a fantastic occasion to view open and globular star clusters, and that’s why Messier 4 makes the perfect object for tonight.

Let’s take a look at how to find it.

Image credit: me, using the free software Stellarium, available at

Those of you — like me — who live at high northern latitudes might be used to using the Big Dipper as your main point-of-reference, following the “arc” of the dipper’s handle to the bright orange giant Arcturus, and then “speeding on” to prominent blue Spica. But from more southerly latitudes, the Big Dipper isn’t a reliable sight; in fact, the famed Southern Cross will be visible in the South at about 10 PM from Mexico tonight!

Instead, you’ll want to find another very bright giant — Antares, the 15th brightest star in the night sky — which can be found tonight (although this is temporary, as planets wander!) by jumping from Mars to Spica to Saturn and then down to Antares, the brightest star in the constellation Scorpius.

Image credit: me, using the free software Stellarium, available at

Although it’s by far the brightest star in its vicinity, there are also a number of naked-eye stars close by. The quite prominent Al Niyat (σ Scorpii), for instance, is just two degrees away back towards Spica, and is bright blue in contrast to the red Antares. If you’re looking with a wide-field eyepiece, a pair of binoculars or a low-powered telescope, Messier 4 can be found roughly in between these two stars, just slightly (maybe 0.5°) south/west of the imaginary line connecting them.

Image credit: me, using the free software Stellarium, available at

And make no mistake about it: this is a spectacular object even through very modest instruments! Messier 4, out of all 29 of the globular clusters identified by Messier himself, is the closest one to us, and the only one Messier was able to resolve into individual stars. He described it thus:

Cluster of very small [faint] stars; with an inferior telescope, it appears more like a nebula; this cluster is situated near Antares & on its parallel.

To Messier, this globular cluster might have looked something like this.

Image credit: Messier 4 at center (with bright Jupiter nearby in 2007), ©2005-2006 Gary Clinch, via

Located a distance of just 7,200 light-years away, this dense cluster of stars has a mass of around 67,000 Suns, and completes an orbit every 116 million years that takes it from just 2,000 light years from the galaxy’s center all the way to 20,000 light years away. Twice per orbit, it passes through the plane of the galaxy, and each time, gravitational interactions likely strip it of some of its stars, meaning it was very likely larger and more massive in the past!

Image credit: N. Tokimasa and H. Naito of Nishi-Harima Astronomical Observatory, via

By looking at the individual stars inside, we find that the amount of heavy elements present — things like Carbon, Oxygen, Silicon and Iron — are far less abundant than they are in our Sun: there’s just 8.5% as much Iron, for example. The reason for this is that globular clusters (like Messier 4) are some of the oldest structures in the Universe, formed very shortly after the Big Bang and stripped of potentially star-forming gas subsequently.

Image credit: © Copyright 2009, Fort Lewis College, Department of Physics & Engineering, via

Messier 4 is no exception, with an estimated age of 12.2 billion years, which explains why there are so few blue-white stars inside. This age means that the stars present inside were formed most recently in an episode occurring just 1.6 billion years after the Big Bang, when the Universe was only 12% its current age, and what remains today is what’s left after more than twice the age Sun and over 100 orbits of this cluster, where it passed through the galactic plane an estimated 210 times!

Image credit: Rolf Wahl Olsen 2011, via

Because this cluster is so old, you might expect there would be a large number of white dwarfs and neutrons stars inside; remnant from prior generations of stars that burned through all of their fuel and died many billions of years ago. Not only are there pulsars (spinning neutron stars) located inside, but Messier 4 is one of the most prolific known sources of white dwarf stars. Thanks to the Hubble Space Telescope, we’ve been able to identify huge numbers of them scattered among the other stars inside.

Image credit: NOAO / AURA / NSF, Hubble image courtesy of NASA / ESA / STScI.

This is only possible because of Messier 4’s close proximity to us! With tens of thousands of stars in a region of space just 75 light years in diameter, this is actually one of the less concentrated globular clusters around, but that doesn’t mean what’s actually inside isn’t spectacular!

Image credit: NASA and H. Richer (University of British Columbia), via

Imagine, if you will, a pulsar (spinning neutron star) orbiting a white dwarf, with a huge gas giant planet 250% the mass of Jupiter in there as well. This exact system has been discovered (and is located in the green circle, above), and is known as PSR B1620-26. The white dwarf inside that circle is 13 billion years old, meaning it likely pre-dates the final episode of star formation that occurred in this globular!

Whether you take in the entire cluster in a wide-field view or just narrow in on the core area, there’s a spectacular beauty in any dense, globular cluster of stars that’s unlike any other structure in the Universe.

Image credit: ESO (top), ESA/Hubble & NASA (bottom).

Normally, I’d show you the Hubble image last, as it’s normally the most spectacular. But in the particular case of this globular cluster, its present location near Antares, a red giant that’s creating its own nebula of gas around it, and Al Niyat, a blue giant blowing off gas as well, means that wide-field views highlighting the spectacular color differences simply blow all other views away.

Image credit: Ivan Eder of, via APOD at

This is one of the brightest, nearest globulars visible from Earth, and I can think of no better way to celebrate the night of cinco de Mayo than with this spectacular deep-sky wonder! And if you need more targets to tickle your fancy, why not take a look back at all the previous Messier objects we’ve examined:

Travel the Universe with astrophysicist Ethan Siegel. Subscribers will get the newsletter every Saturday. All aboard!

Come back again next week for another deep sky wonder and another tale of a piece of our nearby Universe, only here on Messier Mondays!

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