How to see it
Bluish Acrux, otherwise known as Alpha Crucis, is the brightest star in the constellation Crux the Southern Cross. It is the 13th brightest star in all the sky. Far south of the celestial equator, this star cannot be seen from much of North America. Observers north of central Florida or south Texas are out of luck altogether.
For anyone south of about 27 degrees north latitude, Acrux and the Southern Cross exhibit their midnight culmination (highest elevation above the southern horizon) in late March amd early April. The farther south the better, and from approximately the latitude of Brisbane, Australia, the star becomes circumpolar and can be seen every night of the year.
Two nearby and brighter stars, Alpha Centauri and Beta Centauri (Hadar), are known as the Southern Pointers. A line drawn from Alpha through Beta, at about 3 times the distance between them, leads to the top of the Southern Cross, Theta Crucis. Acrux is the bottom of the cross. For an observer facing south, Acrux is the star nearest the horizon. At magnitude 0.77, Acrux is the most southernly first magnitude star. Nearby Mimosa, or Beta Crucis, at magnitude 1.25, is the 19th brightest star in all the heavens.
History and Myth
Unlike many star names, which are Arabic, Latin or Greek proper names, Acrux is simply a combination of “A” (for Alpha) and Crux as the name of the constellation. Not of classical derivation at all, it was a name coined by Elijah Hinsdale Burritt, a Connecticut farm boy turned celestial cartographer, circa 1835.
While little if any mythology is known to be associated with this star, and not much more with the entire constellation, the reputation of the Southern Cross is widespread and in reality, quite exaggerated. The Northern Cross, an asterism of the main stars of Cygnus, the Swan, is larger and better shaped as a Christian Cross, although it contains no stars as bright as Acrux. Interestingly, however, the brightest star in Cygnus, Deneb, is virtually the same apparent magnitude as Mimosa, the second brightest star in Crux. Both are bright blue stars of magnitude 1.25, ranking as the 19th (Mimosa) and 20th (Deneb) brightest stars in the sky.
The reputation of Crux, of which Acrux is the primary star, is as exaggerated as were the rumors of Mark Twain’s death well before his actual demise. In fact, Twain wrote of the Southern Cross, viewed for the first time and reported in his book, Following the Equator:
“We saw the Cross to-night, and it is not large. Not large, and not strikingly bright. But it was low down toward the horizon, and it may improve when it gets up higher in the sky. It is ingeniously named, for it looks just as a cross would look if it looked like something else. But that description does not describe; it is too vague, too general, too indefinite. It does after a fashion suggest a cross — a cross that is out of repair – or out of drawing; not correctly shaped. It is long, with a short cross-bar, and the cross-bar is canted out of the straight line.
“It consists of four large stars and one little one. The little one is out of line and further damages the shape. It should have been placed at the intersection of the stem and the cross-bar. If you do not draw an imaginary line from star to star it does not suggest a cross – nor anything in particular.
“One must ignore the little star, and leave it out of the combination – it confuses everything. If you leave it out, then you can make out of the four stars a sort of cross – out of true; or a sort of kite – out of true; or a sort of coffin-out of true.”
Science
Using data from the Hipparcos mission. Acrux is classified as B0.5 IV. meaning that it is hotter, brighter, larger and more massive than our sun. The designation “IV” indicates that Acrux is a “sub giant” star, not big enough to be considered a giant, but one that has left the realm of normal stars (the “main sequence”) and has entered the terminal phases which ultimately will end up as a white dwarf star.
Acrux is about 321 light-years from Earth, and in fact it is not one star but two nearly identical B-class stars. These can be resolved in a small telescope, making Acrux a nice double, but they appear as a single star to the unaided human eye. The brighter of the two — call it Alpha-1, is magnitude 1.33 and the dimmer — call it Alpha-2 –1.73, with a resultant magnitude of 0.77. Alpha-1 is a subgiant, whereas Alpha-2 appears to still be on the main sequence, rather oddly termed a “dwarf” star. (All stars on the main sequence are technically considered “dwarf stars” to distinguish them from subgiants and giants — there are no “normal” stars!) According to Dr. James Kaler, the subgiant boasts a temperature of about 25,000 Kelvin at its surface, whereas the smaller star is also cooler, about 16,000 Kelvin.
To make things even more complicated, it appears that Alpha-1 is in fact a double star, whose components’ combined masses are about 24 times that of the sun. Alpha-2 is a single star about 13 times as massive as the sun.
Acrux’s position is RA: 12h 26m 35s, dec: -63° 05′ 57″.
Other sources state that Acrux is the twelfth brightest star in the sky, and your article contradicts this. Would you please justify your data to me?
Well, I do not know about justifying anything, but the bottom line is that it is a matter of definition and protocol. Acrux is a multiple star, whose brightest component alone would be (individually) the 20th brightest star in the heavens. Many older references considered only this one component, even though the actual visual appearance of Acrux is the result of at least two stars’ contributions. As such, in some older catalogs, Acrux (just the one brightest component) was listed as the 20th brightest star. However, most modern references use the integrated magnitude of all components in the Acrux system, which gives it a combined visual apparent magnitude of 0.77, just one one-hundredth of a magnitude fainter than Altair, the 12th brightest star. Thus, Acrux is the 13th brightest star visually seen from Earth.
In addition, there have always been some slight controversies over specific magnitudes, including for example that of Betelgeuse, which is variable. At any rate, authorities such as Dr. James Kaler of the university of Illinois, and the modern Hipparcos catalog, list Acrux as the 13th brightest visual star.
Some references — such as Wikipedia — still list outdated information. Keep in mind that we are considering only visual apparent magnitudes — how bright the star appears to the human eye on Earth. In reality, there are stars much brighter than Acrux, but which appear dimmer due to much greater distance.
Dear Mr Sessions
I am especially interested in Acrux-13 since it is the star associated with the year I was born 1945. If you count the number of letters in the alphabet you get the number 13 is M. What I have done with the twenty-eight brightest stars is to connect them with the calendars. Since our calendar has a twenty-eight year cycle you can place all the years in a cycle and use the alphabet to make it easy to identify your own star. It is easy if you know the starting year for each cycle. Let me know if you are interested in finding out what star is associated with your birth year.
Respectfully yours
Ken Kirschner
Ken,
Thank you for your offer. It appears that you have put a lot of thought into it. It is a good mental exercise, especially looking for or developing patterns and relationships. In fact it is an absolutely essential part of science. There is little doubt that it is an integral aspect of human nature, hardwired into our brains. Especially when I was younger, I experienced things such as David DeSalvo discusses in his blog entry here: http://trueslant.com/daviddisalvo/2010/04/12/im-a-pattern-youre-a-pattern-wouldnt-you-like-to-be-a-pattern-too/
On the other hand, EarthSky exists to be a “clear voice for science,” and calendrical associations such as you describe really have nothing to do with science, as far as I can tell. In fact, I would be concerned that some people might associate it with astrological-style or supernatural significance, which is just exactly opposite of what science strives for. As a hobby, it can be fun (I’ve always enjoyed reading about the Chinese astrology — I was born in the “Year of the Rabbit” for example — and other forms of supernatural or paranormal claims. While I try to remain open-minded, I regret to say that I see no convinced data or even argument in favor of any of it). That said, I commend you on your search for patterns, but I do not see any validity of associating stars with particular dates. Perhaps if you could give me a logical, scientific basis for this, I would be more positive, but as for now I will pass. Still, thanks for contacting me and please feel free to ask questions.
Larry Sessions
I could not help but notice the Southern Cross and acrux – a remarkable “star” reflecting many hues – the instant I set eyes on the Southern sky for the first time in Mauritius.
I am curious now as to how close in proximity the stars are that form acrux. Are they part of a shared system?
Wonderful article.
Deborah, the stars in Crux are generally several hundred light years away from Earth, and not, as far as I know, part of a system or gravitationally bound together. However, several of the bright stars (Alpha, Beta and Gamma), due bear similarities in terms of motion and age, and could well have been formed at about the same time from the same large cloud of gas and dust, which likely also spawned many other of the stars we see in this region. This is known as the Scorpius-Centaurus OB Association. “Scorpius-Centaurus” signifies the two larger constellations in which most of the stars of a large “OB Assocation” are located. The “OB” means that they are hot, young, blue or white stars.
Thanks for the reply. I suspected as much that although they appear close, there’s no gravitational relationship. Although you are correct that the reputation of the Southern Cross is a bit exaggerated, the “twinkling” effect resulting from the visual overlapping of these hot stars is remarkable and unique. I appreciated your comments as well about the OB Association stars and their age – a very nice footnote about the geography of the stars in the region.
I noticed a very bright light in the South and in an area where we have constant airplane and helicopter activity so I thought at first this was a plane of some kind; but the “plane” did not move at all and I was stunned by the brightness of this object which I think is Acrux or part of The Southern Cross. I am not an astronomer so I do not know what this object is but it is gorgeous, clear and extremely bright. I have now seen this star on two nights at 5:00a.m. in California
Samuel,
Sorry, but you can’t see Acrux or the Southern Cross from anywhere in California. However, at that time the planet Venus is in the southeastern sky and is very bright as you describe.
Larry Sessions