# What's the difference of the oldest light and newest light?

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Excuse an amateur question, as inspired by this question: "How old is the oldest light visible from Earth?", how to determine the age of light? Does light get old? Does light have characteristic therefore we know this light "A" is different from that light "B" therefore we can track how it going over time? Like tagging animals to observe their activities? How to differentiate one light from the other?

According to that context, the old light means the light that travelling from far away in order to reach the observer, and vice versa.

To determine the age of light, standard method is to use spectral features (absorption/emission but we will talk about the example of emission only without the loss of generality).

Let's say that the observer detects a photon which he is very sure that it must be H$alpha$ emission (i.e., 656.28 nm at rest frame). But, the observer detects a 700.00-nm photon instead of at that rest-rame wavelength. This is because the photon travelling through the universe and experience time dilation; in general, the longer time travelling, the bigger the observed wavelength is, relative to the rest-frame value. Since we have the relationship $lambda_{rest} * (1 + z) = lambda_{obs}$ where $lambda_{rest}$ ($lambda_{obs}$) is the rest-frame (observe-frame) wavelength, and $z$ is the redshift, we can find the redshift from the known rest-frame and observe-frame wavelengths. Then, the higher redshift means farther away, means older.

There is no measurable difference between an "old" photon and a "new" one. A photon has a wavelength, a polarisation, and a direction of travel. And no other features.

You may ask if it is meaningful to discuss the age of light. It is certainly possible to say that photons were emitted by a certain body 100 years ago, and these photons are being detected now, and so this light is 100 years old. But it is impossible to measure the age as a property of the light.

We don't know of any characteristic that uniquely specifies the "age" of the light, so we just assume it's travelled from as far away as we think the object that generated it is, and call the time to travel that far it's "age". In the case of the Cosmic microwave background, this was (we think) released when the universe first became transparent, a few hundred thousand years after the "big bang". The light from this far back has been "stretched" by the expansion of the universe, but not changed by time.

This is normally true of many simple objects. They either exist or not. A proton doesn't age. A neutron may decay, but this is an event, not a process. It is only with complex objects made of many particles that they can change with age while maintaining their identity.

## New View of Nature’s Oldest Light Adds Fresh Twist to Debate Over Universe’s Age

From a mountain high in Chile’s Atacama Desert, astronomers with the United States’ National Science Foundation’s Atacama Cosmology Telescope (ACT) have taken a fresh look at the oldest light in the universe. Their new observations plus a bit of cosmic geometry suggest that the universe is 13.77 billion years old, give or take 40 million years.

The new estimate matches the one provided by the standard model of the universe and measurements of the same light made by the Planck satellite. This adds a fresh twist to an ongoing debate in the astrophysics community, says Simone Aiola, first author of one of two new papers on the findings posted to arXiv.org. In 2019, a research team measuring the movements of galaxies calculated that the universe is hundreds of millions of years younger than the Planck team predicted. That discrepancy suggested that a new model for the universe might be needed and sparked concerns that one of the sets of measurements might be incorrect.

“Now we’ve come up with an answer where Planck and ACT agree,” says Aiola, a researcher at the Flatiron Institute’s Center for Computational Astrophysics in New York City. “It speaks to the fact that these difficult measurements are reliable.”

The age of the universe also reveals how fast the cosmos is expanding, a number quantified by the Hubble constant. The ACT measurements suggest a Hubble constant of 67.6 kilometers per second per megaparsec. That means an object 1 megaparsec (around 3.26 million light-years) from Earth is moving away from us at 67.6 kilometers per second due to the expansion of the universe. This result agrees almost exactly with the previous estimate of 67.4 kilometers per second per megaparsec by the Planck satellite team, but it’s slower than the 74 kilometers per second per megaparsec inferred from the measurements of galaxies.

“I didn’t have a particular preference for any specific value — it was going to be interesting one way or another,” says Steve Choi of Cornell University, first author of the other paper posted to arXiv.org. “We find an expansion rate that is right on the estimate by the Planck satellite team. This gives us more confidence in measurements of the universe’s oldest light.”

Among the scientists from 41 institutions in seven countries who formed part of this collaboration are Professor Kavilan Moodley and Dr Matt Hilton from the University of KwaZulu-Natal (UKZN). Hilton explained that there have been disagreements between methods examining either the oldest light in the universe or galaxies relatively close by to determine the current rate of expansion of the universe, with this result drawn from completely independent measurements of the oldest light in the universe by ACT and Planck finding the same expansion rate. This gives some confidence that there is not a systematic error in these types of measurements.

“This could point to an unidentified problem with measurements of the expansion rate from observing the nearby universe, or it could be the case that both types of measurement are in fact correct – which would mean that there is something missing in our understanding of how the universe evolves,” said Hilton.

Like the Planck satellite, ACT peers at the afterglow of the Big Bang. This light, known as the cosmic microwave background (CMB), marks a time 380,000 years after the universe’s birth when protons and electrons joined to form the first atoms. Before that time, the cosmos was opaque to light.

If scientists can estimate how far light from the CMB traveled to reach Earth, they can calculate the universe’s age. That’s easier said than done, though. Judging cosmic distances from Earth is hard. So instead, scientists measure the angle in the sky between two distant objects, with Earth and the two objects forming a cosmic triangle. If scientists also know the physical separation between those objects, they can use high school geometry to estimate the distance of the objects from Earth.

Subtle variations in the CMB’s glow offer anchor points to form the other two vertices of the triangle. Those variations in temperature and polarization resulted from quantum fluctuations in the early universe that got amplified by the expanding universe into regions of varying density. (The denser patches would go on to form galaxy clusters.) Scientists have a strong enough understanding of the universe’s early years to know that these variations in the CMB should typically be spaced out every billion light-years for temperature and half that for polarization. (For scale, our Milky Way galaxy is about 200,000 light-years in diameter.)

ACT measured the CMB fluctuations with unprecedented resolution, taking a closer look at the polarization of the light. “The Planck satellite measured the same light, but by measuring its polarization in higher fidelity, the new picture from ACT reveals more of the oldest patterns we’ve ever seen,” says Suzanne Staggs, ACT’s principal investigator and the Henry deWolf Smyth Professor of Physics at Princeton University.

As ACT continues making observations, astronomers will have an even clearer picture of the CMB and a more exact idea of how long ago the cosmos began. The ACT team will also scour those observations for signs of physics that doesn’t fit the standard cosmological model. Such strange physics could resolve the disagreement between the predictions of the age and expansion rate of the universe arising from the measurements of the CMB and the motions of galaxies.

“We’re continuing to observe half the sky from Chile with our telescope,” says Mark Devlin, ACT’s deputy director and the Reese W. Flower Professor of Astronomy and Astrophysics at the University of Pennsylvania. “As the precision of both techniques increases, the pressure to resolve the conflict will only grow.”

UKZN plays an essential role through its contribution to characterising the telescope’s on-sky response, and in providing software used to find astrophysical sources and galaxy clusters in the ACT maps. Hippo, UKZN’s high performance computing facility, was used for some aspects of the ACT analysis. Researchers at UKZN’s Astrophysics Research Centre, including a number of postgraduate students and postdocs supported by the National Research Foundation, are using South Africa’s MeerKAT and SALT telescopes to conduct follow-up studies of galaxy clusters detected in the ACT sky maps.

ACT is supported by the National Science Foundation and contributions from member institutions.

### Polarization by Reflection and Scattering

A beam of unpolarized light can be polarized by passing it through a material such as H-sheet. But polarization occurs in certain natural situations as well. When unpolarized light is reflected from a transparent material such as glass or water, the reflected light is partially polarized. Or when sunlight is scattered by air molecules in the atmosphere, the scattered light is partially polarized. The polarizing effect is particularly strong when sunlight is scattered through 90° ( Fig. 14-13 ). On a clear day, when there is little water vapor or dust in the air, and if the Sun is near the horizon, the light from overhead can be polarized to the extent of 70 percent.

FIGURE 14-13 . Unpolarized light from the Sun becomes almost completely polarized when scattered through an angle of 90° by molecules in the atmosphere.

On a sunny day, we often see the glare of reflected light from surfaces of water or glass. Have you ever looked at such surfaces through polarizing sunglasses? (These sunglasses consist of H-sheet sandwiched between two pieces of glass and darkened to absorb even more of the light.) The glare is noticeably reduced, sometimes eliminated almost completely. The reason is that the reflected light consists primarily of overhead light that is reflected horizontally to reach your eyes. The electric vectors in such light are polarized in the horizontal direction and if you use polarizing glasses designed to transmit vertically polarized light , much of the glare is suppressed.

Polarizing glasses are also beneficial to motorists in reducing highway glare and the glare of reflected light from the windshields of approaching automobiles. These glasses are often used by boating and water-sports enthusiasts to reduce water glare. Because the light reflected from water is highly polarized, fishermen sometimes use polarizing glasses to eliminate the reflection, thus enabling them to see into the water more clearly and to locate fish.

Polarized light is sometimes used in engineering design problems to analyze the stresses in structural members. If polarized light is passed through an unstressed transparent material, the analyzed light will show no particular features. However, if the material has internal stresses due to external forces or to the particular method of manufacture, the light analyzed by a piece of H-sheet will show patterns that indicate the regions of stress. In this way, areas of possible structural failure can be identified and corrective measures taken. An example of this kind of analysis is shown in the photograph at the left.

Photographs of reflected glare with (top) and without H-sheet.

Polarized light is used to analyze the stresses in the arches of the famous Gothic cathedral at Chartres, France. A 1:180 model of a typical buttress section was constructed from plastic and then loaded in the way that would result from the prevailing wind conditions at Chartres. This photograph was taken with the polarized light that passed through the plastic model. The regions of stress are clearly shown by the interference patterns.

## What's the difference of the oldest light and newest light? - Astronomy

Some go back to big bang, others live and die many times offer portrait of stellar evolution

### Written by Bob Sheldon

A stronomers peering into the heavens see stars as old as the universe itself and stars recycled time and time again from dust and gases in the interstellar medium.

"Stars are born as fiery cauldrons, about three-quarters hydrogen and one-quarter helium," said Kam-Ching Leung, UNL professor of physics and astronomy. "As they age, their outer envelopes expand, consuming energy generated by fires burning in their cores so lustily that their very atoms are transformed from the original hydrogen to atoms of increasingly complex structure and weight.

"Low-mass stars consume themselves at a slower rate than more massive stars. In its entire lifetime, the conversion of a small-mass star's elemental fuel may go no farther than helium. More massive stars bum hotter and faster, and may end their lifetimes with a core fueled by elements as heavy as carbon or iron." "We look at a star and see how its chemistry changes," Leung said. "The best theory we have now is the 'Big Bang' theory. All of the original stars in the universe were created at about the same time. How long they live depends on their conversion rates, how fast nuclear fusion changes their fuel from one chemical to another. The conversion rate is dependent on high orders of interior temperature, and temperature is strongly dependent on mass.

All stars are in galaxies in a universe of unimaginable dimensions. Our planet orbits a sun that is one of billions of stars in the Milky Way, a galaxy in a group of galaxies that occupies only about onemillionth of the observable universe.

All galaxies are moving away from one another. They have been since the universe was created. "When we look at a galaxy we are looking at its past history," Leung said. "When we look beyond galaxies near to us through our telescopes, we are seeing galaxies as they existed long before our time. The light from the most remote galaxies we can see left those galaxies billions of years ago, and is probably light nearly as old as the universe itself.

Astronomers, therefore, see stars not as bright, twinkling objects of the here and now, but as glimpses into a past stretching back 10 billion years or more. And they know that each and every star they see offers a view of a different stage of stellar evolution, like a frame-by-frame movie that began with creation and ends in cataclysmic chaos.

It is an incomplete panorama, however a film with many blank frames, offering a jerky, primitive motion picture with too may roles in its cast played by unknown actors.

To study a galaxy, Leung said astronomers have to take what they call look-back time into consideration. "When we look at the sun, we don't see the sun as it is now. We see the sun as it was eight minutes ago. When we look at a galaxy, we don't see the galaxy as it is today, because light from the stars in that galaxy may have taken billions of light years to reach earth."

The yardsticks used by astronomers to determine how many light years away is a star or a galaxy are not highly reliable, because when astronomers look through their telescopes they view a universe in two dimensions. They see objects of different brightness and luminosities at vast, but not easily determined, distances from one another. Therefore, it's hard to tell if the dot they see is an asteroid, a comet, a star, or even an entire galaxy.

To compare one galaxy with another, and to peer to the horizon of the universe itself, astronomers need better "measurement yardsticks," according to Leung. Those "yardsticks" are developed as the result of accumulating knowledge about the evolution of the stars.

What is known about the evolution of stars is learned from information about their physical properties--their brightnesses, luminosities, their masses, sizes and physical properties. From those properties, astronomers have constructed what they believe is a reasonably accurate picture of stellar evolution.

As any star ages, part of its mass evaporates into space, while the remaining mass is squeezed tighter and tighter at the core, according to Leung. Eventually, as the outside envelope continues to expand and evaporate, the core of the star will cool, and the star will become a white dwarf.

"If you were to take a star as big as our sun and squeeze it into an object the size of Earth, it would have a density corresponding to our sun if it were a white dwarf," Leung said. "A cubic inch of this white dwarf material would weigh 10 tons.

Our sun, six to seven billion years or so down the road, will become such a white dwarf. A different fate awaits larger stars, however. "Every star's destiny is a product of its mass," Leung said.

"Stars with masses many times larger than our sun's will not only burn hotter and faster as they age, but their cores, whose density increases more and more as its atoms are converted from ever heavier elements, will become squeezed to densities far more compressed than that in a small star such as our sun.

(inset) The evolution of a star: A cloud of gases, mostly hydrogen, condenses to form a star. The circles, clockwise from upper left, indicate the star's evolution: Hydrogen atoms fuse into helium atoms, and the core of the star starts to shrink as helium accumulates at the center. Helium fusion slows and heavier elements are formed one after another. The temperature at the core increases. When iron is formed and collects at the core of the star, an implosion occurs (indicated by inward pointing arrows), in which the star collapses rapidly and violently. The implosion is followed by a tremendous explosion (indicated by arrows pointed outward), in which the star's matter rebounds to produce a supernova .

Heavier elements beyond iron are created in the high temperatures resulting from implosion and explosion of a star. The original core of the star, highly compressed and small in size, becomes a neutron star, or, eventually, a black hole.

"The death of a massive star is far less peaceful than that of a small star, Leung said. "The collapse of the lighter elements into a small star's core becomes a very fast collapse in a star whose core is carbon or iron. What happens is that as the star's outer envelope ex-pands rapidly while the core collapses rapidly, a vacuum is developed at the interface of the core and the envelope. The result is comparable to what happens when a building is demolished. An implosion occurs matter begins hurtling itself inward to the intensely hot core of the star.

"An imploding star is a very dangerous place to be," Leung said. "It's like pouring gasoline on a fire. The material rushing to the core fuels an explosion a high energy action coupled with an equal and opposite reaction that serves to compress the core of the star even more. The stellar envelope is ejected into space during the explosion. There are binary stars whose periods are even less than that, and these end stage of a star whose core mass has core are so tightly squeezed that they stars, with periods measured in seconds, contracted to something almost dimenbreak apart into neutrons."

Thus, a neutron star is formed. A neutron star, Leung said, is not a star at all. It is a core squeezed into so small a size that a mass like our sun could be squeezed into about half the distance between Lincoln and Omaha.

Leung said that there exist in the universe binary star systems that have undergone all of thoses different varieties of evolution, but revolove around a common center of mass, just as our planet and the sun revolve about a common center of mass. (This is the more accurate description of the relationship between our planet and the sun--Earth does not revolve about the sun, but Earth and the sun revolve about a common center mass.)

"In a binary system, the length of the binary orbital period depends on the separation, or distance, between them, Leung said. "If one star in the system is a white dwarf, the separation between the two can be very small, or the two stars can come in contact. When this happens, the rotation of one around the other can be less than a quarter of a day. If both are white dwarfs, the period could be measured in minutes.

There are binary stars whose periods are even less than that, and these stars with periods measured in seconds, are likely to be neutron stars, according to Leung.

There are other things that can happen to stars in their evolution. From the implosion-explosion of a neutron star comes a masive ejection of matter that produces a super nova. Super novas probably account for the manufacture of all the other elements in the universe beyond iron, Leung said. "Gold, silver, radium, uranium and all the other elements are cureated in the debris during a super nova explosion. Since the explosion only lasted for a very short time, the abundance of elements with atomic weights greater than iron is less and these elements are relatively rare."

Incredibly, in view of the intense concentration of mass in a neutron star, even more intense concentration can occur. There are some stars so large, with cores so tightly concentrated, that the horrendous pressures at their core are such that what those cores contain aren't even neutrons. The masses of these stars are squeezed so tight that their total mass can be concentrated into the size of a point on the tip of a ball point pen, Leung said. These are black holes, which Leung said represents the end stage of a star whose core mass ha contracted to something almost dimensionless whose density is incalculable.

To an astronomer, all of ehse natural occurrences in the universe offer opportunities to study its evolution. "Some of the original low mass stars created in the'big bang' co-exist today with recent generations of stars that have become increasingly contaminated with heavy chemicals as they have died and been reborn, sometimes time and time again," Leung said. It is this co-existence of generations of stars of differnet ages that makes it possible to study the universe."-RES

## Seeing vs. Transparency: What's the Difference?

By: Richard S. Wright Jr. December 11, 2017 8

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Good weather for imaging is about more than just the clouds! Even if it's cloud-free, you'll need to understand if the seeing and transparency are good.

A couple of years ago, I attended the OkieTex star party: great location, great skies. I came about halfway through the week and upon arriving, I spotted a vendor friend.

“How’ve the skies been this week?” I asked. “Really good seeing, but the transparency has been not so great.” Cool, thanks.

A moment later I saw another acquaintance. “I hear the skies have been so-so, so far”, I said in greeting.

“Yeah”, he responded, “Really great transparency here, but the seeing last night after the storm was pretty bad.”

The Clear Sky Chart shows the weather forecast for both seeing and transparency as well as cloud cover.

One of these friends did not know the difference between seeing and transparency, and I now knew nothing of what to expect of the coming night. (Actually, I suspected the second friend had it right, and you will too by the end of this blog.)

I figure if there was some confusion between these two ideas among astronomy vendors, then it's likely widespread in the community at large. Indeed, my experience talking with people at star parties continues to bear this out. These two factors affect your imaging plan in two very different ways, so let’s take a look at what these two terms mean and how they affect your night and imaging strategy.

Poor transparency washes out faint details and reduces contrast.
Richard S. Wright Jr.

First, let’s talk about transparency. Transparency is the opacity of the atmosphere, or how clear it is. Moisture and humidity lower the transparency, as does smoke or other kinds of pollution. It’s not entirely unlike light pollution in that it washes out the fainter details of astronomical targets. In fact, poor transparency typically makes light pollution worse because it scatters the light around instead of letting it escape into space away from your cameras and optics.

At my own dark sky camp, the city of Okeechobee to the south provides a transparency “meter,” if you will. How far the light dome extends into the sky isn’t just a factor of how many lights are burning, but how much moisture is in the air to scatter that light around.

When the transparency is poor, I select brighter objects and will shoot targets only when they are high in the sky, where there is as little pea soup to shoot through as possible. (As long as they're not in the direction of Okeechobee!)

Transparency usually gets better with altitude, because you're looking through less air. That's why high altitudes are prized for observatories and star parties.

Transparency is also usually very good after a rainstorm has come through to clear all of the particulates out of the air. This is reason number one I figured my second friend had it right at the star party.

Seeing, on the other hand, is a measure of atmospheric turbulence. We know that if we take a photo of a fast-moving subject, such as at a sporting event, with a low shutter speed, we'll get a blurry image. So what happens when you have to take a very long dark-sky photo and the stars are jumping all about due to atmospheric turbulence? That’s right, blurry stars and deep sky objects.

Seeing is usually measured in arcseconds, an angular measure that describes distance on the celestial sphere. If the seeing was 4 arcseconds, it means the stars can be expected to dance around inside a circle with a diameter of 4 arcseconds. Seeing of 1 arcsecond is 4 times better and would then yield much smaller, less bloated stars, as well as finer detail on deep sky objects.

Poor seeing handicaps how sharp your images start off before processing.
Richard S. Wright Jr.

Seeing is typically better in places where the geography is very flat. The air masses moving over the land encounter few obstacles and flow more smoothly (sometimes called a laminar flow). This is one reason I love imaging in Florida in the wintertime: it has very good seeing. I have friends out West who moved to the desert to escape city lights, but now they're near mountains. The winds coming over the mountains gets all mixed up like a creek flowing over big boulders, which makes for terrible seeing.

Also, after a front comes through (often accompanied by some rainstorms), the air becomes turbulent for a day or so afterwards. Again, my second friend’s claim that after a storm the transparency was good and the seeing was poor fit this pattern best.

If mountains are good for transparency, but poor for seeing, why are so many observatories located up on big mountains? Because as they say. less is more. Less air at high altitudes yields better transparency as I've said, but at the highest mountains you are also above much of the turbulent air, which mitigates the effects on seeing.

Sometimes I will soldier through poor transparency, and if I take enough exposures and spend the time in post-processing, I can often pull something out that I’ll be proud of. Seeing, on the other hand, is often the real limiting factor. You can only do so much sharpening in post processing before things start to look ridiculous, and so if the images are just too soft, it’s time to go to bed, or start that Netflix marathon.

Major observatories are typically at high altitudes to make the most of transparency and seeing.
Richard S. Wright Jr..

Where to draw the line depends on your own personal tastes — and your image scale. Essentially, if your pixels are small and your focal length is long, the poor seeing will just make your images mushy. On the other hand, if you're using a very short focal length and larger pixels, you can do wide-field images of some bigger objects or constellations, with wild abandon to the seeing conditions. I'll return to the topic of pixel scale next month and talk more about how it relates to your seeing conditions.

## Astronomy & Space Exploration Society

• Post author: Astronomy and Space Exploration Society
• Post published: August 11, 2020
• Post category: Latest News

It may surprise you to know that we can still observe the Big Bang, in a way! In fact, every time you accidentally flip to TV static, you’re watching a fragment of it right there! To find out more about this echo of the spawning of the universe, join us online on Wednesday, August 12 at 6:30PM. From that first, immense explosion to now, Dr. Adam Hincks will be delving into the details of the cosmic microwave background radiation! As always, everyone is welcome!

Lecture Abstract:
How to Measure the Universe’s Oldest Light and What it Tells UsThe cosmic microwave background (CMB) is the glow of the
universe from soon after the Big Bang. Today, we can observe this nearly 14 billion-year-old light with microwave telescopes and use it to determine some of the most fundamental properties of the cosmos, such as its age, what it is made out of, and how fast it is expanding. We can also learn how the universe behaved in its very first instants. I will introduce this exciting science and describe how we observe the CMB, focusing in particular on the Atacama Cosmology Telescope and the Simons Observatory—the first currently observing and the second under development—located in the north of Chile.

Dr. Adam Hincks is the inaugural holder of the Sutton Family Chair in Science, Christianity and Cultures at U of T’s David A. Dunlap Department of Astronomy & Astrophysics. Dr. Hincks is an ordained Jesuit priest, and is affiliated with both the Vatican Observatory and the Simons Observatory where he researches the CMB

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## Lookback time

By looking at really distance objects, astronomers can look way back in to the past to study the early universe. To help keep things straight, astronomers refer to something called "lookback time". For any object the lookback time is the age of the universe when the light was first emitted. The lookback time for the light we see from the Sun is the age of the universe (

13.7 billion years) minus 8 minutes . not much of a difference for nearby objects. However, more distant objects have more impressive lookback times. Astronomers can study galaxies with lookback times ranging 4 to 1 billion years --- that's between 9 and 13 billion years ago!

## Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2021 June 3
Millions of Stars in Omega Centauri
Image Credit & Copyright: Ignacio Diaz Bobillo

Explanation: Globular star cluster Omega Centauri, also known as NGC 5139, is some 15,000 light-years away. The cluster is packed with about 10 million stars much older than the Sun within a volume about 150 light-years in diameter. It's the largest and brightest of 200 or so known globular clusters that roam the halo of our Milky Way galaxy. Though most star clusters consist of stars with the same age and composition, the enigmatic Omega Cen exhibits the presence of different stellar populations with a spread of ages and chemical abundances. In fact, Omega Cen may be the remnant core of a small galaxy merging with the Milky Way. Omega Centauri's red giant stars (with a yellowish hue) are easy to pick out in this sharp, color telescopic view.

## Light Gathering Power of Telescopes

This past June, I was accepted to the Astronomy in Chile Educator Ambassador Program and had the amazing opportunity to travel to Chile to learn about the astronomical research being done there.

Our group of 9 consisted of astronomy writers, amateur astronomers, astrophotographers, science teachers, astronomy educators and planetarium professionals from across the United States and from Chile. Together, we make up the 2017 ACEAP team. Each of us brought a love of astronomy and an affinity for communicating that love. The trip also served as an opportunity for us to connect with one another and learn from one another a way to combine our efforts to better spread knowledge of astronomy to the public. No doubt, we were all looking forward to seeing the southern hemisphere night sky, learning about the intriguing astronomical research being done in Chile, and most of all, sharing our experiences with our communities when we return.

Something that I experienced that I want to share with everyone is the immense size of the telescopes! The light gathering power they possess is monumental compared to what our eyes can see. It is for this reason that research telescopes keep getting bigger and bigger. The more light they gather, the deeper we can see into our universe’s past.

The most important property is a telescope’s light gathering power . Today’s research telescopes maximize this important property. The larger the aperture (the opening at the top of the telescope tube), the more light the telescope will gather. These large ‘light buckets’ are collecting photons of light. The more photons of light they can gather the better, and the bigger their aperture, the finer detail they can resolve in very distant objects.

To get a feel for what light gathering power means, let’s start with our eye. What is the light gathering power of your pupil? Let’s figure it out!

The light gathering power is proportional to the area of the main mirror of the telescope. To compare the difference in the light gathering power of our eye to different sizes of telescopes, you calculate the ratio of the areas of their main mirrors (objective lenses).

The mathematical equation reads like so:

Our human pupil has a maximum diameter of about 8 millimeters in dim light.

Let’s compare the light gathering power of our human eye to the size of the telescope mirrors we use in my classes.

The Funscope mirrors have a diameter of 76 millimeters.

The largest telescope mirror at Reimers Observatory, our 25″,

has a diameter of 635 millimeters.

The Gemini telescope I visited in Chile has a mirror with a diameter of 8000 millimeters.

Plug those numbers into the equation for light gathering power and compare them to the light gathering power of our human eye and this is what you get:

The Funscopes have about 90 times the light gathering power that the human eye.

The largest telescope at Reimers Observatory has 6,300 times the light gathering power than the human eye.

The Gemini telescope has 1,000,000 times more light gathering power than the human eye!

I wanted to introduce the idea of light gathering power to my students and also wanted them to experience what it was like to be in the presence of such large mirrors that can gather that much light, so I made a model of the Gemini telescope mirror, located on Cerro Pachón adjacent to the Cerro Tololo Inter-American Observatory in Chile, to use in my classes:

They really enjoyed it! They were blown away by the size of the mirror. Lots of them asked if we could use this exact mylar emergency blanket version like a telescope. That was a perfect opportunity to explain why we couldn’t and why telescope mirrors need to be precise, smooth and also not easily moved by the wind, like the mylar version you see here.

I can’t wait to bring this to more programs so people can appreciate the work that goes into attempting to peer into the deepest reaches of our incredible universe!