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I have read that the lifetime of stars is much less than the period of rotation for a spiral galaxy. For example https://www.coursera.org/learn/evolvinguniverse/lecture/wlYhz/the-origin-of-spiral-arms the professor uses the existence of young blue stars whose creation is triggered by a persistent density wave to claim that the stars lifetime is less than the rotation period. I am not satisfied with this. Given the enormous timescale, how do we know this?
We know how long stars take to orbit in a spiral galaxy, because we know how big galaxies are (we can measure the size by knowing the distance using various methods such as Cepheid variables, and type Ia supernovae) and we know how fast the stars are moving. (We can measure the velocity of stars using the doppler method)
We know how long stars last for because we have a good model of how stars work. We can use this model to predict that large, hot stars will burn themselves out in a short time. We can verify this model by looking at the numbers of blue stars, red supergiants and supernovae.
From the first part we know that stars take about 250 million years to complete an orbit of the Milky way (which is typical for similar sized galaxies) From the second part, we know that giant blue stars live for only tens of million years, and so could not complete a full orbit of the milky way.
On the other hand, smaller stars like the sun live for billions of years and so make multiple orbits.
The ages of sun-like stars
"A Spin-Down Clock for Cool Stars from Observations of a 2.5-Billion-Year-Old Cluster," Søren Meibom, Sydney A. Barnes, Imants Platais, Ronald L. Gilliland, David W. Latham & Robert D. Mathieu, Nature, 517, 589, 2015. The cluster NGC 6819, whose stars are about 2.5 billion years old. Astronomers have measured the rotation periods of thirty Sun-like stars in this cluster and used them to refine the calibrations used to infer a star's age from its rotation period.
The mass of a star is perhaps its most significant feature. It determines how brightly it shines (a star ten times more massive than the Sun will, during its normal lifetime, shine about forty million times brighter than a star ten times less massive than the Sun), how long it will live (tens of millions of years versus tens of billions of years, respectively, for these two cases), and how it will eventually die (as a supernova or as slowly cooling clump of ashes). The next most significant property of a star is its age, which fixes its current character, the age of its planetary system, and the evolutionary state of its environment, and moreover which can be used to refine details in the theory of how stars evolve.
Unfortunately the ages of the most common stars—the modest-mass, cool stars like the Sun and smaller—are difficult to obtain. Traditional dating methods use stellar properties that either change very little as the stars ages or else are hard to determine. Rotation provides an important alternative. Stars rotate (the Sun rotates once approximately every 26 days), and astronomers know that the rotation rate of a cool star decreases with time. Rotation can provide a reliable determinant of stellar age if it can be properly calibrated, in particular across a range of stellar masses. Stars in clusters make perfect reference objects because they all apparently have a similar age. Such age calibrations have indeed, been done, but so far only for stars in clusters less than about one billion years old, not older. This is in part because young stars lose their spin very rapidly as they age, primarily via magnetically powered winds that carry away angular momentum, and after about 600 million years there is a well-defined mass-rotation relationship. The Hyades cluster of stars is about this age, and it has been used to fix the rotation parameters for this young age group. At the older end of the calibration sequence the Sun, at 4.6 billion years, has a well known rotation period. What has been missing is accurate rotation information for the ages in between.
CfA astronomers Soren Meibom and Dave Latham, along with four colleagues, have determined the rotation period measurements for 30 cool stars in the 2.5- billion-year-old stellar cluster NGC 6819. They used the precise data from the Kepler exoplanet mission to monitor cool stars in this cluster, supplemented by ground-based and other datasets. They find a well-defined relationship between rotation period and stellar mass, and make the case that stellar ages can now be determined with a precision of order 10% for large numbers of cool Galactic field stars. This important new age-dating calibration will enable astronomers to study how astrophysical phenomena involving cool stars evolve over time, and will be important to a wide range of research from the Galactic scale down to the scale of individual stars and their companions.
Black holes and galaxies evolve together
In previous studies, scientists have noticed an unexpected proportional relationship between the mass of a supermassive black hole at the center of a large galaxy, which can grow up to billions of times more massive than the sun, and the mass of the galaxy's central area (known as a "bulge"). The proportionality of the masses is especially unusual considering that galaxies and black holes are so different in size, with the bulge generally being orders of magnitude larger. This led the researchers to conclude that galaxies and black holes developed together through coevolution, which involved some physical interaction courtesy of the galactic wind.
As ALMA's press release explains, a galactic wind starts coming into existence when a supermassive black hole gobbles up giant quantities of matter. It is then moved at such a high speed by the black hole's gravity that it radiates intense energy, which in turn, pushes surrounding matter away, creating the galactic wind.
Takuma Izumi, the paper's lead author and a researcher at the National Astronomical Observatory of Japan (NAOJ), says an important question is: "When did galactic winds come into existence in the universe?" Finding this out can lead to understanding how galaxies and supermassive black holes coevolved.
Super Spirals Spin Faster Than Expected, Astronomers Say
Super spirals are the most massive star-forming disk galaxies in the Universe. They are much larger, brighter, and more massive than our own Milky Way Galaxy. Dr. Patrick Ogle of the Space Telescope Science Institute and his colleagues have found that these enormous galaxies spin faster than expected for their mass, at speeds up to 350 miles per second (570 km/sec) for comparison, the Milky Way — an average spiral galaxy — spins at a speed of 130 miles per second (210 km/sec).
The top row of this mosaic features Hubble images of three spiral galaxies, each of which weighs several times as much as the Milky Way. The bottom row shows three even more massive spiral galaxies that qualify as super spirals, which were observed by the ground-based Sloan Digital Sky Survey. The galaxy at lower right, 2MFGC 08638, is the most massive super spiral known to date, with a dark matter halo weighing at least 40 trillion Suns. Image credit: NASA / ESA / P. Ogle & J. DePasquale, STScI / SDSS.
Super spirals are very luminous — they can shine with anywhere from 8 to 14 times the brightness of our Milky Way Galaxy.
They are also giant and massive, with a diameter up to 450,000 light-years and stellar mass between 30 and 340 billion solar masses.
Only about 100 super spiral galaxies are known to date.
“Super spirals are extreme by many measures. They break the records for rotation speeds,” Dr. Ogle said.
Dr. Ogle and co-authors analyzed data for 23 super spiral galaxies collected with the Southern African Large Telescope (SALT), the 5-m Hale telescope of the Palomar Observatory, and NASA’s Wide-field Infrared Survey Explorer (WISE).
They found that these galaxies significantly exceed the expected rotation rate.
They think that the rapid spin of super spirals is a result of sitting within an extraordinarily massive halo of dark matter.
2MFGC 08638, the largest super spiral studied by the team, resides in a dark matter halo weighing at least 40 trillion times the mass of our Sun.
“It appears that the spin of a galaxy is set by the mass of its dark matter halo,” Dr. Ogle said.
The fact that super spirals break the usual relationship between galaxy mass in stars and rotation rate is a new piece of evidence against an alternative theory of gravity known as Modified Newtonian Dynamics (MOND).
This theory proposes that on the largest scales like galaxies and galaxy clusters, gravity is slightly stronger than would be predicted by Newton or Einstein. This would cause the outer regions of a spiral galaxy, for example, to spin faster than otherwise expected based on its mass in stars.
MOND is designed to reproduce the standard relationship in spiral rotation rates, therefore it cannot explain outliers like super spirals. The super spiral observations suggest no non-Newtonian dynamics is required.
Despite being the most massive spiral galaxies in the Universe, super spirals are actually underweight in stars compared to what would be expected for the amount of dark matter they contain. This suggests that the sheer amount of dark matter inhibits star formation.
There are two possible causes: (i) any additional gas that is pulled into the galaxy crashes together and heats up, preventing it from cooling down and forming stars, or (ii) the fast spin of the galaxy makes it harder for gas clouds to collapse against the influence of centrifugal force.
“This is the first time we’ve found spiral galaxies that are as big as they can ever get,” Dr. Ogle said.
Despite these disruptive influences, super spirals are still able to form stars. Although the largest elliptical galaxies formed all or most of their stars more than 10 billion years ago, super spirals are still forming stars today.
They convert about 30 times the mass of the Sun into stars every year, which is normal for a galaxy of that size. By comparison, our Milky Way forms about one solar mass of stars per year.
The findings were published in the Astrophysical Journal Letters.
Patrick M. Ogle et al. 2019. A Break in Spiral Galaxy Scaling Relations at the Upper Limit of Galaxy Mass. ApJL 884, L11 doi: 10.3847/2041-8213/ab459e
The Origin of Cosmic Rays
7.4 Other sources of cosmic rays
Ginzburg suggests that ordinary novae may manufacture cosmic rays: as they each release ∼ 10 −4 of the energy of a supernova, but occur at ∼ 100 y −1 , their total energy output may be a half that of supernovae, so perhaps their cosmic-ray output is in similar proportion.
Acceleration may take place at the galactic centre, where the radio emission is stronger (though perhaps this only means the magnetic field is higher). In the strong extra-galactic radio sources, there is general evidence of violent emission of material from the nuclear region, as though explosive activity in galactic nuclei is not uncommon, and many normal galaxies show radio emission mainly from a central region. Lacking quantitative data on this aspect of our galaxy at present, we cannot assess this possibly very important source in the same way as supernovae.
Burbidge and Hoyle (1964) have urged that cosmic rays may be largely of extra-galactic origin, arguments by Burbidge and counterarguments by Ginzburg and Syrovatskii having continued at the successive international conferences. Of course, if cosmic rays leak out of our galaxy at 10 41 erg s −1 , they presumably do likewise from most others, and with a density of average galaxies ∼ 10 −75 cm −3 ( Allen, 1963 ) leaking steadily for 10 10 y, one only has from this source (allowing for red-shifts) an energy density ∼ 1·5 × 10 −5 of that seen near the Earth. The strong radio-galaxies which may spill particles at 10 3 times the rate of normal ones (if the relativistic particles escape in ∼ 10 6 y) have a space density ∼ 2 × 10 −79 cm −3 , so may only double the rate. It is believed that weak magnetic fields exist even outside galaxies, and particles would hence follow irregular paths, wandering only a few tens of Mpc even at 10 18 eV according to Ginzburg and Syrovatskii so one may consider the supposed “local supergalaxy” to be the limit of the region of interest, where the density of average galaxies may be at least 10 times higher. One may then guess an extragalactic flux ∼ 3 × 10 −4 of the observed general flux.
The air shower spectrum above 10 18 eV may be taken to indicate an extragalactic flux ⩾ 1 100 of the galactic flux, in which case a factor ⩾ 30 is to be accounted for. Radio source counts have indicated much greater activity in the past than at present, corresponding to a very high output when galaxies were young (e.g. Longair, 1966 ), and hence such a factor might be retrieved, if we take it that these extra-galactic rays are very old. But then interactions with the supposed far infrared (3°K) photons in intergalactic space should have reduced the flux appreciably above 10 18 eV and very greatly above 5 × 10 19 eV. So the position is not yet clear.
One gains the impression from Figure 15 (p. 61 ) that there is a separate component of cosmic rays below 30 MeV, with a very steep spectrum. Since at 2-3 MeV the range of a proton is < 0·01 g cm −2 , their source is probably fairly local, perhaps some other kind of star. Or, the solar system may simply admit slow particles more readily.
Dark matter tugs the most massive spiral galaxies to breakneck speeds
The top row of this mosaic features Hubble images of three spiral galaxies, each of which weighs several times as much as the Milky Way. The bottom row shows three even more massive spiral galaxies that qualify as “super spirals,” which were observed by the ground-based Sloan Digital Sky Survey. Super spirals typically have 10 to 20 times the mass of the Milky Way. The galaxy at lower right, 2MFGC 08638, is the most massive super spiral known to date, with a dark matter halo weighing at least 40 trillion Suns. Astronomers have measured the rotation rates in the outer reaches of these spirals to determine how much dark matter they contain. They found that the super spirals tend to rotate much faster than expected for their stellar masses, making them outliers. Their speed may be due to the influence of a surrounding dark matter halo, the largest of which contains the mass of at least 40 trillion suns. Credit: Top row: NASA, ESA, P. Ogle and J. DePasquale (STScI). Bottom row: SDSS, P. Ogle and J. DePasquale (STScI)
When it comes to galaxies, how fast is fast? The Milky Way, an average spiral galaxy, spins at a speed of 130 miles per second (210 km/sec) in our Sun's neighborhood. New research has found that the most massive spiral galaxies spin faster than expected. These "super spirals," the largest of which weigh about 20 times more than our Milky Way, spin at a rate of up to 350 miles per second (570 km/sec).
Super spirals are exceptional in almost every way. In addition to being much more massive than the Milky Way, they're also brighter and larger in physical size. The largest span 450,000 light-years compared to the Milky Way's 100,000-light-year diameter. Only about 100 super spirals are known to date. Super spirals were discovered as an important new class of galaxies while studying data from the Sloan Digital Sky Survey (SDSS) as well as the NASA/IPAC Extragalactic Database (NED).
"Super spirals are extreme by many measures," says Patrick Ogle of the Space Telescope Science Institute in Baltimore, Maryland. "They break the records for rotation speeds."
Ogle is first author of a paper that was published October 10, 2019, in the Astrophysical Journal Letters. The paper presents new data on the rotation rates of super spirals collected with the Southern African Large Telescope (SALT), the largest single optical telescope in the southern hemisphere. Additional data were obtained using the 5-meter Hale telescope of the Palomar Observatory, operated by the California Institute of Technology. Data from NASA's Wide-field Infrared Survey Explorer (WISE) mission was crucial for measuring the galaxy masses in stars and star formation rates.
Referring to the new study, Tom Jarrett of the University of Cape Town, South Africa says, "This work beautifully illustrates the powerful synergy between optical and infrared observations of galaxies, revealing stellar motions with SDSS and SALT spectroscopy, and other stellar properties—notably the stellar mass or 'backbone' of the host galaxies—through the WISE mid-infrared imaging."
Theory suggests that super spirals spin rapidly because they are located within incredibly large clouds, or halos, of dark matter. Dark matter has been linked to galaxy rotation for decades. Astronomer Vera Rubin pioneered work on galaxy rotation rates, showing that spiral galaxies rotate faster than if their gravity were solely due to the constituent stars and gas. An additional, invisible substance known as dark matter must influence galaxy rotation. A spiral galaxy of a given mass in stars is expected to rotate at a certain speed. Ogle's team finds that super spirals significantly exceed the expected rotation rate.
Super spirals also reside in larger than average dark matter halos. The most massive halo that Ogle measured contains enough dark matter to weigh at least 40 trillion times as much as our Sun. That amount of dark matter would normally contain a group of galaxies rather than a single galaxy.
"It appears that the spin of a galaxy is set by the mass of its dark matter halo," Ogle explains.
The fact that super spirals break the usual relationship between galaxy mass in stars and rotation rate is a new piece of evidence against an alternative theory of gravity known as Modified Newtonian Dynamics, or MOND. MOND proposes that on the largest scales like galaxies and galaxy clusters, gravity is slightly stronger than would be predicted by Newton or Einstein. This would cause the outer regions of a spiral galaxy, for example, to spin faster than otherwise expected based on its mass in stars. MOND is designed to reproduce the standard relationship in spiral rotation rates, therefore it cannot explain outliers like super spirals. The super spiral observations suggest no non-Newtonian dynamics is required.
Despite being the most massive spiral galaxies in the universe, super spirals are actually underweight in stars compared to what would be expected for the amount of dark matter they contain. This suggests that the sheer amount of dark matter inhibits star formation. There are two possible causes: 1) Any additional gas that is pulled into the galaxy crashes together and heats up, preventing it from cooling down and forming stars, or 2) The fast spin of the galaxy makes it harder for gas clouds to collapse against the influence of centrifugal force.
"This is the first time we've found spiral galaxies that are as big as they can ever get," Ogle says.
Despite these disruptive influences, super spirals are still able to form stars. Although the largest elliptical galaxies formed all or most of their stars more than 10 billion years ago, super spirals are still forming stars today. They convert about 30 times the mass of the Sun into stars every year, which is normal for a galaxy of that size. By comparison, our Milky Way forms about one solar mass of stars per year.
Ogle and his team have proposed additional observations to help answer key questions about super spirals, including observations designed to better study the motion of gas and stars within their disks. After its 2021 launch, NASA's James Webb Space Telescope could study super spirals at greater distances and correspondingly younger ages to learn how they evolve over time. And NASA's WFIRST mission may help locate more super spirals, which are exceedingly rare, thanks to its large field of view.
Galaxy formation is at the foundation of modern astrophysics. Studies of stellar motions in the outskirts of galaxies and of the distribution of galaxies in space have shaken the foundations of modern physics and revealed that 96% of the energy composition of the Universe is due to Dark Matter and Dark Energy. The most exotic of astronomical objects — black holes − are routinely found in centers of nearby galaxies and are now thought to have made a profound impact on galaxy formation. The cycle of creation of chemical elements by a galaxy’s generations of stars provides the basis for our ideas about the origins and prevalence of life itself. Explosions, disruptions and mergers of stars and black holes in external galaxies fuel an exciting emerging field of time-domain astrophysics.
The faculty members with primary interests in extragalactic astronomy include Profs. Timothy Heckman, Colin Norman, Nadia Zakamska and Research Prof. William Blair, and of course lines are often blurred between extragalactic astronomy and cosmology. Extragalactic astrophysicists at CAS study galaxy formation from the dawn of the observable universe, including star-bursting galaxies and supermassive black holes in the centers of galaxies. A major strength of the extragalactic astronomy effort at JHU has been our membership and leading role in the cutting-edge wide-area optical surveys: SDSS-I, II, III, IV (currently ongoing) and Pan-STARRS, and Sumire / PFS in the near future. But extragalactic astronomy is a truly multi-wavelength enterprise, so astrophysicists at CAS use a multitude of ground-based and space-based facilities across the entire electro-magnetic range. For example, using the Chandra X-ray Observatory to obtain some of the deepest images of the X-ray sky ever made, University Professor and Nobel Laureate Riccardo Giacconi and Prof. Colin Norman showed that the diffuse X-ray background is in fact composed out of a large number of accreting supermassive black holes in galaxy centers. In the paragraphs below we highlight a small subsample of other recent results from our extragalactic astronomy groups.
Hopkins professor receives lifetime achievement award for contributions to field of astrophysics
Johns Hopkins astrophysicist Tim Heckman has been awarded the 2018 Catherine Wolfe Bruce Gold Medal by the Astronomical Society of the Pacific, a lifetime achievement award that recognizes outstanding contributions to astrophysics research.
Since its founding in 1889, the ASP has become a leading nonprofit astronomy organization dedicated to bringing together professionals, amateurs, educators, and enthusiasts for the purpose of increasing the understanding of astronomy. Established and named for a 19th century American philanthropist and patroness of astronomy, the Catherine Wolfe Bruce Gold Medal was first awarded in 1898 to Hopkins mathematician Simon Newcomb. Other recipients have included astronomical pioneers Giovanni V. Schiaparelli, Edwin Hubble, Fred Hoyle, and Vera Rubin.
Image caption: Tim Heckman
Heckman, chair of the Department of Physics and Astronomy in the Krieger School of Arts and Sciences, works to understand galaxy formation, active galactic nuclei, and the relationship between galactic evolution and the life cycle of the super massive black holes at their cores. He discovered that nuclear activity is ubiquitous in so-called normal galaxies, an early foreshadowing of the later discovery that nearly all galaxies have supermassive black holes in their centers.
A renowned astrophysical observer, Heckman has helped analyze data from the Sloan Digital Sky Survey, the Hubble Space Telescope, and the Galaxy Evolution Explorer. From his observations, Heckman has published several definitive papers on galaxy evolution that are considered by many to be benchmarks of the field, including a paper establishing that galaxies undergoing intense episodes of star-formation drive powerful winds that affect the evolution of galaxies in profound ways. He and his colleagues have shown how the stellar populations and structure of galaxies correlate with the galaxy mass, determined which types of galaxies are home to supermassive black holes that are most rapidly growing, and demonstrated that the chemical composition of galaxies is tightly correlated with galaxy mass. Heckman also directs the Johns Hopkins Center for Astrophysical Sciences.
Heckman is a fellow of the American Academy of Arts and Sciences and a member of the National Academy of Sciences.
Prediction and detection potential of fusion neutrinos from nearby stars
Kai Zuber , Santiago Arceo Díaz , in Astroparticle Physics , 2020
2 Nearby stars
The “ solar neighborhood ” was constrained to a distance of 12 parsecs, a distance just large enough to include at least one star from the most luminous spectral classes. Both factors, spectral class and distance are important since the neutrino production rate grows larger with stellar luminosity (as most neutrinos are being produced by thermonuclear reactions) and decreases with the square of the distance. Within the proposed solar neighborhood there are around 526 stars  , out of which 350 are red dwarfs, that would conform a background neutrinos due to their low intrinsic production and large number, and 34 are white dwarfs, only producing KeV-thermal neutrinos  . Any potential source should be strong enough as to surpass the collective contribution of all the dim red dwarfs, otherwise it could be impossible to identify the incoming direction of the detected neutrinos (see last section). The remaining potential sources correspond to 143 stars belonging to the K, G, F and A spectral classes.
The Eggleton numerical code was used to simulate the most representative stellar spectral classes within the solar neighborhood, its functioning, characteristics and updates are described in [22–25] . The total output of neutrinos, considered for the numerical models, includes thermonuclear and thermal reactions  . The concordance between the numerical models and data found in the literature was tested by comparing the central temperature and density of the standard solar models described in [28,29] to the values predicted by an stellar track, with M i = 1 M ⊙ , Y i = 0.28 , Z = 0.02 and α = 2.0 , evaluated when it reached the solar bolometric luminosity. The values found (Tc = 15.76, in units of 10 6 K, and ρ c = 1.60 , in units of 10 2 g cm − 3 ) differ in less than 1% to the values predicted by the standard solar models ( T c = 15.70 and ρ c = 1.62 ). The numerical models, used to represent the most promising neutrino stellar sources, estimate a bolometric luminosity, stellar radius and effective temperature that differ below 5% to the calibrations found by photometric and asteroseismologic studies mentioned in the next section.
An arbitrary threshold on neutrino luminosity was set to further constrain the number of stars that could individually serve as potential identifiable neutrino sources: 10% of the theoretical solar neutrino output. According to the numerical models done in this work, and the empirical bolometric corrections by Habets and Heintze  , any main-sequence star with a visual magnitude MV < 8 can be considered as a relevant, since they surpass the mentioned constraint on neutrino luminosity within the estimated age of the Universe (see the red dotted line in Fig. 1 ). Using the empiric relationship between bolometric luminosity and stellar mass for main-sequence stars, this limit corresponds to a stellar mass around 0.6 M⊙ (the exact value varies slightly with metallicity: M i = 0.63 M⊙ for Z = 0.001 and M i = 0.59 M⊙ for Z = 0.02 ) and rules out all the M dwarfs and most of the main-sequence K stars in the solar neighborhood.
Fig. 1 . Color-magnitude diagram (CMD) of the stars within 12 parsec, separated from the remaining stars in the Gliese catalogue (grey marks). The horizontal line corresponds to the threshold, predicted by stellar models, below which an individual star can not achieve 10% of the solar neutrino luminosity within the age of the Universe.
Fig. 2 . Neutrino spectrum for the stars in our sample (solid lines), compared to the solar neutrino spectrum (dashed lines) from the standard solar model by Bahcall and Pinsonneault  . For comparison’s sake, the neutrino flux was scaled as if each star was located at 1 A.U. The numbers represent the variation on the flux related the variation in initial mass for the optimal and the upper and lower boundary models. For all the stellar models, the flux from each reaction was obtained by scaling the intrinsic neutrino production rates shown in Table 3 . The predicted true neutrino flux at Earth is shown in Table 4 .
How to learn a star's true age
For many movie stars, their age is a well-kept secret. In space, the same is true of the actual stars. Like our Sun, most stars look almost the same for most of their lives. So how can we tell if a star is one billion or 10 billion years old? Astronomers may have found a solution -- measuring the star's spin.
"A star's rotation slows down steadily with time, like a top spinning on a table, and can be used as a clock to determine its age," says astronomer Soren Meibom of the Harvard-Smithsonian Center for Astrophysics.
Meibom presented his findings May 24, 2011 in a press conference at the 218th meeting of the American Astronomical Society.
Knowing a star's age is important for many astronomical studies and in particular for planet hunters. With the bountiful harvest from NASA's Kepler spacecraft (launched in 2009) adding to previous discoveries, astronomers have found nearly 2,000 planets orbiting distant stars. Now, they want to use this new zoo of planets to understand how planetary systems form and evolve and why they are so different from each other.
"Ultimately, we need to know the ages of the stars and their planets to assess whether alien life might have evolved on these distant worlds," says Meibom. "The older the planet, the more time life has had to get started. Since stars and planets form together at the same time, if we know a star's age, we know the age of its planets too."
Learning a star's age is relatively easy when it's in a cluster of hundreds of stars that all formed at the same time. Astronomers have known for decades that if they plot the colors and brightnesses of the stars in a cluster, the pattern they see can be used to tell the cluster's age. But this technique only works on clusters. For stars not in clusters (including all stars known to have planets), determining the age is much more difficult.
Using the unique capabilities of the Kepler space telescope, Meibom and his collaborators measured the rotation rates for stars in a 1-billion-year-old cluster called NGC 6811. This new work nearly doubles the age covered by previous studies of younger clusters. It also significantly adds to our knowledge of how a star's spin rate and age are related.
If a relationship between stellar rotation and age can be established by studying stars in clusters, then measuring the rotation period of any star can be used to derive its age -- a technique called gyrochronology (pronounced ji-ro-kron-o-lo-gee). For gyrochronology to work, astronomers first must calibrate their new "clock."
They begin with stars in clusters with known ages. By measuring the spins of cluster stars, they can learn what spin rate to expect for that age. Measuring the rotation of stars in clusters with different ages tells them exactly how spin and age are related. Then by extension, they can measure the spin of a single isolated star and calculate its age.
To measure a star's spin, astronomers look for changes in its brightness caused by dark spots on its surface -- the stellar equivalent of sunspots. Any time a spot crosses the star's face, it dims slightly. Once the spot rotates out of view, the star's light brightens again. By watching how long it takes for a spot to rotate into view, across the star and out of view again, we learn how fast the star is spinning.
The changes in a star's brightness due to spots are very small, typically a few percent or less, and become smaller the older the star. Therefore, the rotation periods of stars older than about half a billion years can't be measured from the ground where Earth's atmosphere interferes. Fortunately, this is not a problem for the Kepler spacecraft. Kepler was designed specifically to measure stellar brightnesses very precisely in order to detect planets (which block a star's light ever so slightly if they cross the star's face from our point of view).
To extend the age-rotation relationship to NGC 6811, Meibom and his colleagues faced a herculean task. They spent four years painstakingly sorting out stars in the cluster from unrelated stars that just happened to be seen in the same direction. This preparatory work was done using a specially designed instrument (Hectochelle) mounted on the MMT telescope on Mt. Hopkins in southern Arizona. Hectochelle can observe 240 stars at the same time, allowing them to observe nearly 7000 stars over four years. Once they knew which stars were the real cluster stars, they used Kepler data to determine how fast those stars were spinning.
They found rotation periods ranging from 1 to 11 days (with hotter, more massive stars spinning faster), compared to the 30-day spin rate of our Sun. More importantly, they found a strong relationship between stellar mass and rotation rate, with little scatter. This result confirms that gyrochronology is a promising new method to learn the ages of isolated stars.
The team now plans to study other, older star clusters to continue calibrating their stellar "clocks." Those measurements will be more challenging because older stars spin slower and have fewer and smaller spots, meaning that the brightness changes will be even smaller and more drawn out. Nevertheless, they feel up to the challenge.
"This work is a leap in our understanding of how stars like our Sun work. It also may have an important impact on our understanding of planets found outside our solar system," said Meibom.