Polaris Brightness Variations are Revived, Astronomers Mystified
Written by Ian O'Neill
Polaris A (Pole Star) with its two stellar companions, Polaris Ab and Polaris B. Artist impression. Credit: NASA
Polaris is a well known Cepheid variable, but its periodic brightness variations have been steadily decreasing in amplitude for the last hundred years. Around the beginning of the 20th Century, Polaris' brightness fluctuated every four days by 10%. Only ten years ago this variation had dropped to 2%, leading astronomers to believe this steady decline in the variability of the star was about to end. That was until recent observations uncovered an increase in variability to 4%. Polaris is an odd star in that it is a Cephid variable with a declining variability, and now astronomers are baffled as to why the brightness fluctuation has been revived…
Polaris (a.k.a. the North Star or Pole Star) has helped mankind navigate the globe since ancient times. Always positioned around the North Polar axis of the Earth, Polaris has also provided material for literature, poetry and religion. In astronomical terms it is also significant as it is a Cepheid variable with a regular variation in brightness, although it is the only Cepheid variable known that has been decreasing in brightness for the last several decades. But to complicate matters even further, this Type 1a supergiant (approximately 4-5 solar masses and 30 solar radii) appears to have been rejuvenated, and the vibrations have increased, varying in brightness by 4 %.
This discovery comes after observations made by Hans Bruntt from the University of Sydney and his international collaboration. Dr Alan Penny, co-investigator from the University of St. Andrews, UK, will present the team's findings at his university's "Cool Stars 15" conference this week.
In reality, the astronomers had focused their attention on Polaris in the hope to catch the point at which its variations ceased completely, only to find they had increased. "It was only through an innovative use of two small relatively unknown telescopes in space and a telescope in Arizona that we were able to discover and follow this star's recovery so accurately," Penny said. He was using the SMEI space camera, usually applied for solar-terrestrial observations of the solar wind, but he used it to accurately survey the night sky for Cepheid variables. At the same time, Bruntt was using a small telescope attached to NASA's retired infra-red space telescope (WIRE) set up to study Polaris for a short period. When Penny noticed the strange recovery of Polaris in his SMIE data, it was compared with Bruntt's WIRE data. It was therefore confirmed that Polaris' vibrations had been revived.
Decrease over 100 years of amplitude of 4-day light variation of Polaris and of the increase since 2000. Credit: H. Bruntt et al. 2008
Backing up Penny and Bruntt, Professor Joel Eaton (Tennessee State University), who was using the AST automated spectroscopic telescope located in Arizona, noticed variations in the plasma velocity on the surface of Polaris. These measurements showed the brightness variations were correlated with expansion and contraction effects through the body of the star.
These observations are both exciting and perplexing. Although the variations observed in Cepheid variables are poorly understood, the vast majority of these "standard candles" do not change in brightness, let alone revive themselves. It would appear Polaris is undergoing a change that isn't predicted by the standard model for stellar evolution, so the team of astronomers will be quick to follow up these observations with some theory as to what is causing the changes inside Polaris…
Filed under: Astronomy, Observing
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July 23rd, 2008 at 3:33 am
Let me see…dark energy, dark matter. I know! Dark plasma! You can't see it or detect it directly but, by studying how it affects ordinary plasma velocity, its existence can be postulated. Got any more mysteries that need solving?
July 23rd, 2008 at 6:40 am
Either dark matter or it's passing through some serious dusty regions in space. Star brightness doesn't fluctuate with its maximum and minimum cycles, correct? If that's true, then I can rule that out for myself.
July 23rd, 2008 at 6:45 am
No, folks, you all have it wrong. This is actually a signal from an alien species! They know this is our "guiding star", so they're cleverly using it to send us a message. All we have to do is interpret the variations the right way and bingo, we're in the galactic family!
July 23rd, 2008 at 6:49 am
So much for standard candles, they flicker. The instrumentation technology is amazing. Stellar theory, however, is in an abysmal state, with far less than predictive power, and seems to be worse off with every new discovery. The plasma behavior…keeping in mind that this ionized stuff constitutes an electric current…hints at the underlying cause of the variation.
July 23rd, 2008 at 6:50 am
Or a large number of plus jupiter sized planets in its system would create this effect as well…
July 23rd, 2008 at 6:55 am
On a more serious note (forget my alien nonsense…it was for fun), we know that Polaris is a trinary star. I wonder if it's possible the other two stars could be having an effect on the Cepheid's variability…
July 23rd, 2008 at 7:41 am
Close in Jupiters, the companion stars, whatever . . . something simple will turn up.
Statistically, Cepheids are reliable. Remember, this is the exception, not the rule.
July 23rd, 2008 at 8:11 am
But, John, are not the Cepheids said to be varying by some (unknown) intrinsic property, which is the cornerstone of their use as standard candles? Thus the statistically aberrant modification of a Cepheid behavior by unknown external influences should be especially troubling. And we would have to ask why other Cepheids should not vary in response to external influences…or are they already – that would sure make the candles less standard? The exception seems to open a kettle of fish.
July 23rd, 2008 at 3:02 pm
Let's see now, a trinary star has been observed to decrease in amplitude for a mere 100 years and we expected it to fizzle out and not it not to resume it's former variability?
July 23rd, 2008 at 7:11 pm
"Bill Says:
July 23rd, 2008 at 6:49 am
"So much for standard candles, they flicker."
>>>Well, it's the amplitude of the variations that is changing in this case, but it is the period of variations has not, and it is this property that is associated with the star's absolute magnitude and hence is useful for determining distance. The amplitude does not matter in terms of using cepheids as a 'standard candle'. Furthermore, there has never been an exception found to the period-luminosity relationship in cepheids. So from that point of view, this observation changes nothing.
However, it is an interesting find, and obviously one that will need an explanation if we can one day hope to say that we understand stars and stellar behaviour.
Bill Says:
July 23rd, 2008 at 8:11 am
"But, John, are not the Cepheids said to be varying by some (unknown) intrinsic property, which is the cornerstone of their use as standard candles?"
>>>The mechanism of variability is not completely unknown. There is a good deal of evidence to suggest that it is due to the repeated ionisation/expansion/deionisation/contraction cycle of the stellar atmosphere. The star heats helium in its atmosphere and ionises it. This plasma then becomes more opaque due to the ionisation, and radiation pressure pushes the atmosphere outwards and expands it. The atmosphere then cools due to the expansion, deionises and becomes less opaque. The atmosphere contracts, heats up and reionises and the cycle continues.
As for what can modify the amplitude of this cycle, I know not, but it is not to hard to envision some process that could modulate the amplitude variation on lengthy timescales while leaving the period essentially unchanged.
July 24th, 2008 at 5:14 am
Nice job Astrofiend.
Like I stated in a post a few days ago, every once in a while something comes along to mess wtih our constants, and we find the universe isn't the age it was, and distances are different. I wouldn't get too concerned with an outside influence just yet. It may be something passing and unique. In which case we learn how to recognize it and treat it as a signpost which has been turned in the wrong direction.
However, everything isn't just going to be thrown out the window. There may an external reason why this is doing what it is, or it may be a part of its lifecycle we just haven't witnessed before. or we could have missed it totally and we need to get out the tap shoes.
All in all, I love the fact we gain a lot more questions every day than we find answers. Some of us are just sicker than others when it comes to being attracted to the unknown.
July 28th, 2008 at 2:52 am
Hi! there is one another group who has done similar work in polaris…the link is http://arxiv.org/abs/0805.1165….
They both are interesting….
regards,
Vino