We are just beginning to understand how such different kinds of galaxies could arise and evolve. But the picture is blurry. For example, cosmological models of galaxy formation haven’t been able to establish exactly where the stars we see today were first born. Figuring out how galaxies came to be is hugely important. Without them, there would be no stars – and therefore no life in the universe.
Now we have made a discovery, published online in the Monthly Notices of the Royal Astronomical Society, that sheds new light on how galaxies and stars within them form.
Astronomers know that some of the Milky Way’s stars were born in the galaxy itself, whereas others were formed in smaller galaxies that the Milky Way eventually engulfed. But it is unclear which process is most common.
Infant stars are born in large groups – never in isolation. Some of these groups, called clusters, are still around today. They roughly come in two types: young, low-mass clusters that live in the Milky Way disk, and massive, old “globular clusters” which are located in the “stellar halo” of the Milky Way (a very large, thin, diaphanous structure) which envelops the disk of the Milky Way. Globular clusters contain a tiny fraction of all Milky Way stars, but astronomers suspect that they hold important clues to the early stages of how the galaxy formed.
But we don’t really know how globular clusters formed either. Current models propose that most globular clusters formed in giant molecular clouds within turbulent disks in the early universe. They suggest that globular clusters host multiple generations of stars – stars born from material of other stars, some of which are now dead. However, these models cannot reproduce in detail what we actually observe in globular clusters.
What we know is that the universe only assembled the conditions to form lots of globular clusters in its youth. Ironically, these turbulent disks were also hostile to globular clusters – it is thought that most of them were wiped out by collisions with giant molecular clouds shortly after being born. Those that survived (there are about 150 in the Milky Way) may have lost a substantial fraction of their stars, perhaps even most of them. But if that’s the case, then there should be plenty of stars originally formed in globular clusters now residing in other parts of the galaxy.
The answer could be written in the stars themselves. Their locations, velocities and chemical compositions may hold clues to whether these stars were formed in the Milky Way or elsewhere. All the chemical elements in the universe, except for hydrogen, helium, and tiny amounts of lithium, were synthesised entirely in the interior of stars. When stars die, they shed their nucleosynthetic byproducts into the interstellar medium, out of which new stars are born. Thus, a stars’s chemical composition bears the signature of the evolution of its predecessors. So if we can work that out, it may be possible to establish a star’s origin.
The galaxy has many many stars, though, so to obtain meaningful answers, one needs very large samples. Thanks to recent technological developments, huge surveys currently collect data for hundreds of thousands to millions of Milky Way stars. One of them is the Apache Point Observatory Galactic Evolution Experiment (APOGEE). It stands out from the crowd because of its focus on infrared spectroscopy – a lower frequency of light than that we can see – as opposed to all other surveys that concentrate in the visible part of the spectrum.
This is important because of all the dust in the disk of the Milky Way. Dust is a lot less opaque in the infrared, so APOGEE can see through the dust into the inner regions of the Milky Way much better than optical surveys. This enabled us to estimate the abundances of some key elements for thousands of stars for the first time. We were also able to detect trace stellar families that would otherwise pass unnoticed amid a crowd of ordinary stars.
In this way, we discovered a population of stars with unusual chemical compositions, very enriched in nitrogen. This is interesting because it is typical of globular cluster stars. The properties of these new stars differ from those of existing globular clusters, suggesting that they were associated with clusters that no longer exist. The logical implication is that there once existed a very large population of globular clusters in the early Milky Way, which was almost entirely destroyed. Moreover, the properties of the new stars suggest that they are associated with the halo of the Milky Way, not its disk. That being the case, destroyed globular clusters may be the stuff that at least a quarter of the halo is made of.
If confirmed, this result will challenge models of galaxy formation. For instance, APOGEE’s result can constrain which fraction of the halo was formed in the galaxy. It also forces us to revise models of globular cluster formation, which have a hard time explaining the amount of nitrogen that we observe in these stars.
But perhaps a more far-reaching implication is that we might discover that globular clusters are in fact one of the the most typical star-forming units in the universe. In the past two decades, research has made it possible to estimate the mean chemical composition of stars in the cores of distant, unresolved elliptical galaxies, which are thought to be formed in similar ways to the Milky Way halo. Interestingly, this has shown that stars in those galaxies tend to be enriched in nitrogen and sodium, which is precisely what is found in globular clusters.
Indeed, globular clusters may have contributed substantially to the stellar budget of all galaxies in the universe – something we didn’t know before. This is a riveting prospect which could even come to change our understanding of how galaxies came about, including our own Milky Way.
By Ricardo Schiavon, Reader in Astrophysics, Liverpool John Moores University
This article was originally published on The Conversation. Read the original article.
The record of observations over the past 100 years or so begs the question, will this rise continue? A new study that colleagues and I have published in PNAS looks at what happens to the sea level at 2°C warming and beyond. Our work, led by Svetlana Jevrejeva of the UK’s National Oceanography Centre, shows two key things.
First, that for an emissions scenario that causes global temperature to rise by 2°C and 5°C relative to the global temperature around 1870, there are a range of sea levels projected from 53cm to 178cm above the level in 2000. Second, that while the global average is projected to increase with rising temperatures, sea levels will not increase by the same amount everywhere.
Let’s start with the second point as it will help answer the first. The sea is not level. In fact, it is anything but level. The reason is gravity. The force that keeps your feet firmly on the ground is also acting upon the ocean. The ocean is continually adjusting itself so that its surface – the sea level – feels the same gravitational pull everywhere (called an equipotential surface).
Large things sitting on land, such as ice sheets, are big enough to pull (via their own gravity) the ocean towards them. If bits of an ice sheet or glacier fall off then their ability to pull the ocean towards them is less, so the ocean adjusts, and you have just added water to the ocean that wasn’t in it before which alters the sea level as well.
This adjustment is happening all the time. Around 20,000 years ago there used to be some enormous ice sheets over North America and Scandinavia, which pushed down the land surface in those places. As the ice sheets melted the land surface rebounded and is still doing so today. Further from where these ice sheets used to be, the land subsided and its still doing so today. And there was the small matter of all that melted ice raising global sea level by about 120m, which was adjusting itself to keep gravity equal across its surface.
The ocean itself has currents (which govern the large-scale transport of heat, freshwater, and nutrients) whose strength cause local increases in the height of sea level. The ocean is also warming up. And a warmer ocean causes the sea level to rise since liquids expand as their temperature increases.
Each of the components that contribute to sea level (glaciers, ocean warming, ice sheets and so on) has a unique pattern for how much it will add to sea level in a particular place. If we add them up we get the total sea level at that point and also how much each component has contributed.
This leads us back to the first point, where I said that for each temperature rise there are a range of possible projected sea levels. Scientists have run computer simulations of how different levels of greenhouse gases and aerosols could affect the climate (atmosphere and ocean), the size and shape of glaciers and ice sheets, the amount of water on land that humans can dam and extract, and the way the land and ocean respond to adding and removing ice and water.
The (really really) short result of all this research is that each component shows a range of possible responses to each scenario. By running the computer simulations many times, it appears that certain responses are more likely to happen than others. This allows us to say something about how likely a certain sea-level rise could occur in the future.
This is best shown with an example. Let’s take Jakarta. The Indonesian capital is home to around 12m people, of whom nearly 1.6m live less than 450cm above sea-level.
Our most likely projection for a temperature rise of 5°C is 85cm, which is pretty much the same as the global average. However, we can also think about what all those possible responses of each component are for a 5°C warming. In these situations there is a less than 5% chance of sea level being less than 49cm and less than 5% chance of sea level being more than 180cm.
In the two cases, different components contribute a different amount (remember they each have a unique pattern). In the “lower” projection (49cm) it is ocean warming that dominates, while in the “higher” projection (180cm) it is the Antarctic ice sheet that dominates, followed by not insignificant contributions from ocean warming, glaciers and the Greenland ice sheet.
Of course there are other things going on locally other than these components. For Jakarta, the gravest concern is subsidence. This mega city is sinking, mainly because of groundwater extraction. Projections of subsidence by 2100 for Jakarta are 230-300cm. Put these together with our higher sea level projection and we get 410-480cm of local sea-level rise. Remember the 1.6m people currently who live below 450cm? That number is likely to rise with a growing population.
We started with the phrase “sea-level rise”. It is really important that we unpack this sort of phrase to fully understand what it means. We must recognise the difference between a global number and a local number. For Jakarta, global sea-level is important, but the local sea-level matters too. The sea level is not level – we should accept this fact and embrace the complexity of what makes up rising seas.
By Luke Jackson, James Martin Fellow, The Institute for New Economic Thinking, University of Oxford
This article was originally published on The Conversation. Read the original article.
This semiannual ritual shifts our rhythms and temporarily makes us groggy at times when we normally feel alert. Moreover, many Americans are confused about why we spring forward to DST in March and fall back in November, and whether it is worth the trouble.
The practice of resetting clocks is not designed for farmers, whose plows follow the sun regardless of what time clocks say it is. Yet many people continue to believe that farmers benefit, including lawmakers during recent debates over changing California DST laws. Massachusetts is also studying whether to abandon DST.
Changing our clocks does not create extra daylight. DST simply shifts when the sun rises and sets relative to our society’s regular schedule and routines. The key question, then, is how people respond to this enforced shift in natural lighting. Most people have to be at work at a certain time – say, 8:30 a.m. – and if that time comes an hour earlier, they simply get up an hour earlier. The effect on society is another question, and there, the research shows DST is more burden than boon.
Benjamin Franklin was one of the first thinkers to endorse the idea of making better use of daylight. Although he lived well before the invention of light bulbs, Franklin observed that people who slept past sunrise wasted more candles later in the evening. He also whimsically suggested the first policy fixes to encourage energy conservation: firing cannons at dawn as public alarm clocks and fining homeowners who put up window shutters.
To this day, our laws equate daylight saving with energy conservation. However, recent research suggests that DST actually increases energy use.
Poster celebrating enactment of daylight saving time during World War I, 1917 (click to zoom).
This is what I found in a study coauthored with Yale economist Matthew Kotchen. We used a policy change in Indiana to estimate DST effects on electricity consumption. Prior to 2007, most Indiana counties did not observe DST. By comparing households’ electricity demand before and after DST was adopted, month by month, we showed that DST had actually increased residential electricity demand in Indiana by 1 to 4 percent annually.
The largest effects occurred in the summer, when DST aligns our lives with the hottest part of the day, so people tend to use more air conditioning, and late fall, when we wake up in the dark and use more heating with no reduction in lighting needs.
Other studies corroborate these findings. Research in Australia and in the United States shows that DST does not decrease total energy use. However, it does smooth out peaks and valleys in energy demand throughout the day, as people at home use more electricity in the morning and less during the afternoon. Though people still use more electricity, shifting the timing reduces the average costs to deliver energy because not everyone demands it during typical peak usage periods.
DST proponents also argue that changing times provides more hours for afternoon recreation and reduces crime rates. But time for recreation is a matter of preference. There is better evidence on crime rates: Fewer muggings and sexual assaults occur during DST months because fewer potential victims are out after dark.
So overall, the net benefits from these three durational effects of crime, recreation and energy use – that is, impacts that last for the duration of the time change – are murky.
Other consequences of DST are ephemeral. I think of them as bookend effects, since they occur at the beginning and end of DST.
When we “spring forward” in March we lose an hour, which comes disproportionately from resting hours rather than wakeful time. Therefore, many problems associated with springing forward stem from sleep deprivation. With less rest people make more mistakes, which appear to cause more traffic accidents and workplace injuries, lower workplace productivity due to cyberloafing and poorer stock market trading.
Even when we gain that hour back in the fall, we must readjust our routines over several days because the sun and our alarm clocks feel out of synchronization. Some impacts are serious: During bookend weeks, children in higher latitudes go to school in the dark, which increases the risk of pedestrian casualties. Dark commutes are so problematic for pedestrians that New York City is spending US$1.5 million on a related safety campaign. And heart attacks increase after the spring time shift – it is thought because of lack of sleep – but decrease to a lesser extent after the fall shift. Collectively, these bookend effects represent net costs and strong arguments against retaining DST.
Spurred by many of these arguments, several states are considering unilaterally discontinuing DST. The California State Legislature considered a bill this term that would have asked voters to decide whether or not to remain on Pacific Standard Time year-round (the measure was passed by the State Assembly but rejected by the Senate).
On the East Coast, Massachusetts has commissioned research on the impacts of dropping DST and joining Canada’s Maritime provinces on Atlantic Time, which is one hour ahead of Eastern Standard Time. If this occurred, Massachusetts would be an hour ahead of all of its neighboring states during winter months, and travelers flying from Los Angeles to Boston would cross five time zones.
Countries observing daylight saving time (blue in Northern Hemisphere, orange in Southern Hemisphere). Light gray countries have abandoned DST; dark gray nations have never practiced it.
TimeZonesBoy/Wikipedia, CC BY-SA
These proposals ignore a fundamental fact: Daylight saving time relies on coordination. If one state changes its clocks a week early, neighboring states will be out of sync.
Some states have good reason for diverging from the norm. Notably, Hawaii does not practice DST because it is much closer to the equator than the rest of the nation, so its daylight hours barely change throughout the year. Arizona is the sole contiguous state that abstains from DST, citing its extreme summer temperatures. Although this disparity causes confusion for western travelers, the state’s residents have not changed clocks’ times for over 40 years.
In my research on DST I have found that everyone has strong opinions about it. Many people welcome the shift to DST as a signal of spring. Others like the coordinated availability of daylight after work. Dissenters, including farmers, curse their loss of quiet morning hours.
When the evidence about costs and benefits is mixed but we need to make coordinated choices, how should we make DST decisions? When the California State Senate opted to stick with DST, one legislator stated, “I like daylight savings. I just like it.” But politicians’ whims are not a good basis for policy choices.
The strongest arguments support not only doing away with the switches but keeping the nation on daylight saving time year-round. Yet humans adapt. If we abandon the twice-yearly switch, we may eventually slide back into old routines and habits of sleeping in during daylight. Daylight saving time is the coordinated alarm to wake us up a bit earlier in the summer and get us out of work with more sunshine.
Laura Grant, Assistant Professor of Economics, Claremont McKenna College
This article was originally published on The Conversation. Read the original article.
