Star positions matter

Updating The Chilling Stars

Why star positions matter for climate physics

The Making of History’s Greatest Star Map is an excellent account of the European Space Agency’s Hipparcos mission by the project scientist, Michael Perryman. It brings back vivid recollections:

• of dismay after the launch in 1989, when the satellite failed to go into the right orbit and frantic steps were needed to improvise a survivable orbit and re-configure the observing programme.
• of satisfaction when operations continued despite unplanned exposure to the Earth’s radiation belts, as well as some nasty solar flares, until the radiation damage became fatal in 1993.
• of the appetizer in 1994, when early results of the Hipparcos star mapping helped in accurate prediction of the impacts of the fragmented Comet Shoemaker-Levy 9 on the planet Jupiter.
• of joy on Isola di San Giorgo, Venice, in 1997 when the Hipparcos science team announced their first large-scale results, after a huge computational effort.

Hipparcos in an ESA impression

Astrometry took that great leap forward 30 years after Pierre Lacroute of the Strasbourg Observatory first proposed a space mission to measure the positions of stars, 20 years after Erik Høg of the Copenhagen Observatory refined the concept, and 17 years after ESA earmarked it as something to do. Ground-based astrometry had stalled, because of imprecisions due the turbulence of the atmosphere, and its remaining aficionados had little lobbying power. As a result, Hipparcos remained a distinctly European space project – the first in which there was no competition with the US or Soviet space science programmes.

Applications of the Hipparcos Catalogue of 100,000 plus stars and the Tycho 2 Catalogue with 2.5 million stars (to a lesser but still unprecedented accuracy) have ranged from detecting a bend in the Milky Way Galaxy to checking Einstein’s theory of gravity, General Relativity. But wanting to pursue here the relevance of Hipparcos to climate physics, I’m pleased to see that Michael Perryman points the way.

Michael Perryman. Photo by Richard Perryman

In The Making of History’s Greatest Star Map, pp. 236-243, Perryman notes the role of Hipparcos in refining observations the wobbles of the Earth’s axis, which are involved in the pacing of ice ages (the Milankovitch theory). Then he points to the link between solar activity and climate change, as evidenced by the Little Ice Age, the Medieval Warm Period and other variations. As to the mechanism for the solar connection, Perryman singles out the suggestion that cosmic rays, modulated by solar activity, influence cloud cover.

He continues the story with the Sun’s journey through the Galaxy and the icy intervals on Earth that correspond to exposure to intense cosmic rays when passing through spiral arms. That’s a major topic in The Chilling Stars and, as Perryman says, the Hipparcos data have improved our knowledge of motions in the Galaxy.

It’s reassuring when a professor of astronomy with no scientific or political axe to grind gives serious attention to the cosmic-ray/climate link (the Svensmark hypothesis). Let me reciprocate by reviewing what’s said about the climate-related significance of Hipparcos and its successor Gaia in The Chilling Stars and see if it needs updating or extending.

Did our planet look like Jupiter's icy moon Europa during the Snowball Earth episodes? Image: NASA Voyager 2

Henrik Svensmark and I devote much of our Chapter 6, “Starbursts, tropical ice and life’s changing fortunes”, to the geologists’ discovery of episodes called Snowball Earth. Around 2300 million years ago, and again around 700 million years ago, there were glaciers and icebergs even in the tropics. A strong fit to the Svensmark hypothesis comes with the evidence that these were times of enhanced cosmic rays due to stellar baby booms, or starbursts. Here’s an extract related to Hipparcos and Gaia.

To find out when the star formation rate has changed, astronomers take a census of the stars. If you find in a human population an unusually large number of people in a certain age group, you’ll know that they were born during a baby boom. So it is with stars. But to calculate the age of any star, astronomers have first to measure how far away it is. The distances of many stars became much better known in 1997, with the release of results from Europe’s star-mapping satellite Hipparcos.

Astronomers in Brazil and Finland used the Hipparcos data to help them to compare the ages of some 500 nearby stars. By 2000, Helio Rocha-Pinto and his colleagues were able to report clumps in the ages that told of several stellar baby booms during the Galaxy’s long history. The survivors seen today are necessarily modest, long-lived stars, but their massive cousins would have soon exploded and generated cosmic rays in abundance during those periods of high rates of star formation.

One of the baby booms fell in the period 2,400–2,000 million years ago. An unusual number of stars of the same age in the Small Magellanic Cloud provides supporting evidence and points the finger at a neighbouring galaxy that may have come close enough to provoke the action in the Milky Way. On the other hand, some astronomers suspect that the Large Magellanic Cloud was the perpetrator. Knowledge of the comings and goings of the Magellanic Clouds and other small neighbours in their clumsy dance is sketchy at best. So is the timetable of their close approaches or ‘perigalacticons’. Timings will remain uncertain until better measurements of the present motions of the nearest galaxies become available from Europe’s next star-mapping spacecraft, Gaia, by about 2015.

Meanwhile, what stands out is the correspondence in time between the early Snowball Earth episodes of about 2,300 million years ago and Rocha-Pinto’s starburst in the period 2,400 to 2,000 million years ago. There are reasons for suspecting that the two events were connected, by the unusually high cosmic rays to which the Earth was subjected. But if this was more than a chance coincidence, then the ice-free interval that followed should be associated with a scarcity of stars born at that time. For [Nir] Shaviv [of Jerusalem] this was a key point in his argument.

“The long period of 1 to 2 billion years before present, during which no glaciations are known to have occurred, coincides with a significant paucity in the past star formation rate.”

And the later Snowball Earth episodes starting around 750 million years ago should also be linked to another stellar baby boom. Rocha-Pinto’s census of Hipparcos stars does indeed show star-birth much reduced between 2,000 and 1,000 million years ago. But the rate of star formation that follows the lull, in this census, is not very impressive. More persuasive are the results of another survey announced in 2004 by Raúl de la Fuente Marcos of Suffolk University, Madrid, and Carlos de la Fuente Marcos of Universidad Complutense de Madrid. They used data on groups of stars called open clusters, as catalogued by astronomers over many years, to infer among their other results a starburst around 750 million years ago. The Fuente Marcos pair noted its timeliness for Shaviv’s story.

“The Snowball Earth scenario appears to be connected with the strongest episode of enhanced star formation recorded in the solar neighbourhood during the last 2,000 million years.”

Here is extraordinary support for the idea that cosmic rays have controlled the climate throughout the Earth’s history. When a hypothesis is false, new experiments and observations will tend to quarrel with it, but with a good theory the reverse is true. It looks better and better as the facts become more exactly known.

UPDATES: A small point for The Chilling Stars concerns the expected date for Gaia results. The launch is scheduled for 2012 but the data gathering and processing for more than a billion stars and other objects is a huge task. Although some preliminary results can be expected by 2015, the work won’t be finished until 2020.

What’s chiefly needed here, about Hipparcos, picks up on what the Fuente Marcos duo were doing with open star clusters and folds it back to the previous Chapter 5 “The Dinosaur’s Guide to the Galaxy”. Nearby open star clusters spawned supernovae that showered the Earth with extra cosmic rays throughout the past billion years. They provide another perspective on the variable cosmic rays from the Galaxy due to the Solar System’s peregrinations, as mentioned by Michael Perryman and discussed at length in The Chilling Stars.

Here’s a plot of star clusters and their ages prepared by Henrik Svensmark for a DTU publication using data from the University of Vienna’s WEBDA database, which makes extensive use of Hipparcos results in fixing distances and ages.

Distribution of stellar clusters within 1 kpc (~3300 light-years) of the Solar System in the Galactic plane of the Milky Way Galaxy and up to 1,000 million years old. The colours denote the ages, which indicate when supernovae in the clusters were intensifying the cosmic rays at the Earth. As clusters eventually disperse, most clusters are relatively young. The ringed dot in the centre denotes the position of the Solar System. Source: H. Svensmark, see reference.

Hipparcos next crops up in The Chilling Stars in connection with human evolution, in Chapter 7, “Children of the supernovae?” and attempts to identify the star cluster most likely resonsible for a particularly near supernova about 2.8 billion years ago.

Which squadron of stars included the one that blew up closely enough to splash the Earth with that identifiable trace of iron-60 atoms? The relative positions of the Sun and its violent neighbours have changed during the past few million years. At times the action was closer than it is now. Accurate plotting of distances and motions by the European Space Agency’s star-mapping satellite Hipparcos (1987–93) has helped astronomers to try to figure out where the culprit lay.

A candidate location is roughly in the direction of the Southern Cross, and therefore invisible from Europe or North America. It’s the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus OB association, at present around 400 light-years away. As reckoned by Jesús Maíz-Apellániz at Johns Hopkins University in Maryland, the sub-group was almost 100 light-years closer just a few million years ago. One of its outlying stars might have come within 120 light-years and then blown up.

Otherwise the gang of stars responsible may lie on the other side of the sky, in the constellation of Taurus. The Pleiades, mentioned earlier as the Seven Sisters, are the most famous cluster of them all. Although not in the ring of Gould’s Belt, they may share a common origin. They are close enough for your naked eye to see several bright blue members, out of the cluster’s complement of about a hundred stars. The distance to the Pleiades is increasing, so they were previously nearer still. The largest OB stars of the cluster are no longer there,

because they have already exploded. Perhaps twenty have done so in as many million years. In Germany, Thomas Berghöfer of the Hamburg Observatory and Dieter Breitschwerdt of the Max-Planck-Institut für Extraterrestrische Physik in Garching have proposed that one of the missing Pleiades was responsible for the splash of iron-60 atoms.

Deciding the issue may have to wait for more detections of the exotic radioactive atoms, both in the sky and on the Earth. For the time being, the closeness of the Munich supernova is a continuing puzzle for astronomers. Both of those suggestions – Lower Centaurus-Crux and the Pleiades – could turn out to be wrong.

UPDATE: For this passage there’s already a update at the end of an earlier post: https://calderup.wordpress.com/2010/05/16/2-75-million-years-ago/#more-806 and I repeat it here.

In The Chilling Stars we make much of the radioactive iron-60 atoms deposited on the Earth by a nearby supernova. A puzzle is to work out which of the nearby clusters of young, exploding stars may have been responsible. We rashly suggested the best-known of all the clusters, the Pleiades or Seven Sisters, but now we realise that the last of their massive, explosive siblings blew up 100 million years ago.

Another candidate is the Lower Centaurus-Crux sub-group of the Scorpius-Centaurus OB association, but that seemed too far away – more than 300 light-years away 2-3 million years ago. The suggested limit for the source of the iron-60 was 120 light-years, if the radioactive atoms were to survive their journey to the Earth, so we could only suggest a possible outlier of the cluster.

But in an interesting development the half-life of iron-60 has been drastically revised, to make them more durable. A report by a German-Swiss team (see the Rugel reference) increases the half-life from 1.49 to 2.62 million years. The authors comment:

Furthermore, a supernova deposit of ⁶⁰Fe on Earth, assumed some 2 to 3 Myr ago, should now yield a lower value at that time because of the longer half-life. Hence a more distant source must be assumed.

This makes the Lower Centaurus-Crux sub-group a more convincing candidate.

Henrik Svensmark and I are grateful to Dr Rainer Facius for drawing this to our attention.

Postscript to that update.

For knowing astronomers: this judgment about the Pleiades follows WEBDA data and it isn’t closely related to a well-known dispute about the Hipparcos distance to the Pleiades, widely believed to be too small.

For more about the link between climatic effects of cosmic rays during human evolution see https://calderup.wordpress.com/2010/05/16/2-75-million-years-ago/

In a final passage, Gaia figures prominently in Chapter 8 of The Chilling Stars, “The agenda for cosmoclimatology.”

The most important project of all, for the astronomical contribution to cosmoclimatology, is Europe’s Gaia space mission. It’s the successor to Hipparcos, which mapped the brightest stars far more accurately than ever before. Better-defined distances to the stars provided the means of gauging their ages, and so discovering the ‘baby booms’ in star formation that turned out to be linked to the extreme cold of the Snowball Earth episodes. But the use of a small sample of stars, and remaining uncertainties in the Hipparcos measurements, means that the star-formation story is still sketchy, with the census taken at intervals of 400 million years. What’s more, the analysis so far is confined to our local domain in the Milky Way, in a region of the disc far removed from the Galaxy’s centre. Gaia will far surpass Hipparcos in the accuracy and scope of its star measurements and in its reach across the Galaxy. The multinational team aims to tell the entire story of star formation in the Milky Way over more than 10 billion years, in the central bulge, in all the different rings of the disc, and in the surrounding halo of stars. Only then will there be a clear answer to the question: ‘Was star formation relatively smooth or highly episodic?’ The more episodic it was, the greater will be the scope for looking for effects of high cosmic-ray influxes on the Earth.

Better knowledge of the spiral arms will be another payoff. Hipparcos gave quite a good map of the local Orion Arm, where the Sun is now. Gaia will map all the major arms on the near side of the Galaxy, by locating the newly formed stars that populate them. The high-precision measurements should also give a firm answer for the rate at which the spiral pattern revolves around the galactic centre, and whether the Sun’s orbit is a circle or an ellipse. The results will make possible more exact calculations of when the Sun and its planets visited the spiral arms and experienced the high cosmic rays responsible for the icehouse periods in geological history.

Until Gaia, no great improvement in our knowledge of the star formation rate during the Earth’s history can be expected. The spacecraft is not due to fly until 2011 and its observations will take about five years to complete. Meanwhile, there is plenty of information from astrophysics, including observations of other spiral galaxies, for theorists to chew over. They can try to improve, for example, their grasp of the contrasts in galactic magnetism and cosmic-ray fluxes between the bright spiral arms and the darker inter-arm regions.

UPDATE: Repeating what was said above about Gaia’s timeline: the launch is now scheduled for 2012 but the data gathering and processing for more than a billion stars and other objects is a huge task. Although some preliminary results can be expected by 2015, the work won’t be finished until 2020.

On the mapping of spiral arms: that part of the project becomes more important at a time when there is disagreement even about how many arms the Galaxy has – four main spirals or only two? Meanwhile other spacecraft, including NASA’s Fermi gamma-ray satellite are giving relevant new views of the Galaxy, including the diffuse gamma-ray background due to cosmic rays.

References

M. Perryman, The Making of History’s Greatest Star Map, Springer-Verlag 2010

H. Svensmark & N. Calder, The Chilling Stars, Icon 2007 and 2008

H. Svensmark, “The Adventurous Journey of Spaceship Earth”, Technical University of Denmark DTU Yearbook, 2009

G. Rugel et al., “New Measurement of the ⁶⁰Fe Half-Life”, Physical Review Letters, Vol. 103, pp. 072502 1-4, 2009

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