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A Brief Tour of the Universe

Question: Can you provide a brief tour of the major objects visible in the universe?

Sure. If you look out at the nighttime sky, what you see right away are a few bright things that are planets in our own solar system or possibly the Moon, which is a terrible thing. We hate to have the Moon because all that reflected light makes the sky bright; makes it hard to do astronomy for distant objects. So, the real black belt astronomers don’t like the Moon; don’t like the planets very much. But, if you go out at night, you’ll see stars. And the stars that you see emitted their light, tens or hundreds, or even thousands of years ago. The speed of light, which everybody thinks is so fast, is really extremely slow. And it’s what lets astronomers see back into the past. 

So, for example, the speed of light is a foot, that’s a unit of distance, used here in the United States and I believe also in Myanmar. It’s a foot in a nanosecond, or a billionth of a second. So, you never see things the way they are, you always see the past. You always see light that bounced off somebody 10 nanoseconds ago, or in the back of a big room a hundred nanoseconds ago. When you go outside, you’re seeing light that was emitted – the sunlight that was emitted eight minutes ago, light in the solar system maybe up to an hour ago. And when you look at the stars, even the nearby stars, even the bright stars, the light has been traveling to your for tens of years, or hundreds of years, or even thousands of years. So, without a telescope you can see in the past a few thousand years. And what happened in the 1920’s was that people began to realize that the system of stars that we are in, which is the Milky Way; the Milky Way Galaxy we call it today, which we see as a band of light in the summer sky because we’re looking at this system, which is a big flattened system; kind of like a pizza edge on, except we’re a pepperoni. We’re on the pizza. And so our view of it is really quite awkward. We don’t have a good perspective of the Milky Way Galaxy. But we know now that it’s roughly speaking 100,000 light years in dimension across our Milky Way. So that means it takes light 100,000 years to travel across that span of distance. And that’s really just the beginning.

What people discovered in the 1920’s was that our galaxy is just one of billions of galaxies out there. The distances between the galaxies are a few times their own diameter. So, if the galaxy is this big, then the distance to the next galaxy is kind of ten times the size of the galaxy. So, if this is 100,000 light years, the distance to the next galaxy is a few million light years. And that’s fairly accurate. The galaxy that you can see – there’s one galaxy you can see without a telescope, if you know where to look, with binoculars. And easy object in a small telescope, and that’s the Andromeda Galaxy, M31. And in the autumn sky you can pick it out. It’s kind of a fuzzy patch. What we know is that is as big a system as the one we live in. It’s as big as the Milky way; it looks like a little tiny fuzzy patch because it’s so far away. 

And that’s really just the beginning. That’s our local neighborhood a few million light years away. It turns out that with modern telescopes and the best instruments and the better detectors we have today, it’s not that hard to see things that are a few billion light years away, or to measure the light from them anyway. So, that’s a thousand times farther away, it means the objects appear a million times dimmer. But what has changed over time is that we have big telescopes that collect a lot of light, and we have detectors that are nearly perfect at measuring the light and turning it into an electronic signal. So, very similar to the detectors that are in digital cameras and so on; they are made of silicon they work pretty much the same way, but we take long time exposures and we add up the data very carefully. 

Anyway, we’re able to make this – the technology has enabled us to make this leap so that we can study the distant objects. And the reason why we want to do that is that the telescope is really a king of no nonsense time machine. It let’s you see the way things were in the past. Of course, it doesn’t let you see into the future. It only lets you see the past, but we can do that to distances of a billion light years, and even with some effort, to many billions of light years. And that’s important because the time since the beginning of cosmic expansion, since the beginning of the universe as we know it, the time of the Big Bang, we think is about 14 billion years ago. So the biggest distance that light could travel in that time is about 14 billion light years. And we can see things most of the way back. That means we’re not just guessing that the universe has changed over time. The universe has expanded over time; has it gotten elaborated over time due to the action of gravity pulling stuff together and stars making more complicated elements and all that stuff that’s happened over the past 14 billion years is not just a story, it’s a real history that we can observe.

Recorded on February 17, 2010
Interviewed by Austin \r\nAllen


What’s floating around out there in the cosmological zoo? The Harvard astronomer describes the major objects visible via telescope and the naked eye.

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  • The massive star in the Kinsman Dwarf Galaxy seems to have disappeared between 2011 and 2019.
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  • Maybe it's still there, but much less luminous and/or covered by dust.

A "very massive star" in the Kinman Dwarf galaxy caught the attention of astronomers in the early years of the 2000s: It seemed to be reaching a late-ish chapter in its life story and offered a rare chance to observe the death of a large star in a region low in metallicity. However, by the time scientists had the chance to turn the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Paranal, Chile back around to it in 2019 — it's not a slow-turner, just an in-demand device — it was utterly gone without a trace. But how?

The two leading theories about what happened are that either it's still there, still erupting its way through its death throes, with less luminosity and perhaps obscured by dust, or it just up and collapsed into a black hole without going through a supernova stage. "If true, this would be the first direct detection of such a monster star ending its life in this manner," says Andrew Allan of Trinity College Dublin, Ireland, leader of the observation team whose study is published in Monthly Notices of the Royal Astronomical Society.

So, em...

Between astronomers' last look in 2011 and 2019 is a large enough interval of time for something to happen. Not that 2001 (when it was first observed) or 2019 have much meaning, since we're always watching the past out there and the Kinman Dwarf Galaxy is 75 million light years away. We often think of cosmic events as slow-moving phenomena because so often their follow-on effects are massive and unfold to us over time. But things happen just as fast big as small. The number of things that happened in the first 10 millionth of a trillionth of a trillionth of a trillionth of a second after the Big Bang, for example, is insane.

In any event, the Kinsman Dwarf Galaxy, or PHL 293B, is far way, too far for astronomers to directly observe its stars. Their presence can be inferred from spectroscopic signatures — specifically, PHL 293B between 2001 and 2011 consistently featured strong signatures of hydrogen that indicated the presence of a massive "luminous blue variable" (LBV) star about 2.5 times more brilliant than our Sun. Astronomers suspect that some very large stars may spend their final years as LBVs.

Though LBVs are known to experience radical shifts in spectra and brightness, they reliably leave specific traces that help confirm their ongoing presence. In 2019 the hydrogen signatures, and such traces, were gone. Allan says, "It would be highly unusual for such a massive star to disappear without producing a bright supernova explosion."

The Kinsman Dwarf Galaxy, or PHL 293B, is one of the most metal-poor galaxies known. Explosive, massive, Wolf-Rayet stars are seldom seen in such environments — NASA refers to such stars as those that "live fast, die hard." Red supergiants are also rare to low Z environments. The now-missing star was looked to as a rare opportunity to observe a massive star's late stages in such an environment.

Celestial sleuthing

In August 2019, the team pointed the four eight-meter telescopes of ESO's ESPRESSO array simultaneously toward the LBV's former location: nothing. They also gave the VLT's X-shooter instrument a shot a few months later: also nothing.

Still pursuing the missing star, the scientists acquired access to older data for comparison to what they already felt they knew. "The ESO Science Archive Facility enabled us to find and use data of the same object obtained in 2002 and 2009," says Andrea Mehner, an ESO staff member who worked on the study. "The comparison of the 2002 high-resolution UVES spectra with our observations obtained in 2019 with ESO's newest high-resolution spectrograph ESPRESSO was especially revealing, from both an astronomical and an instrumentation point of view."

Examination of this data suggested that the LBV may have indeed been winding up to a grand final sometime after 2011.

Team member Jose Groh, also of Trinity College, says "We may have detected one of the most massive stars of the local Universe going gently into the night. Our discovery would not have been made without using the powerful ESO 8-meter telescopes, their unique instrumentation, and the prompt access to those capabilities following the recent agreement of Ireland to join ESO."

Combining the 2019 data with contemporaneous Hubble Space Telescope (HST) imagery leaves the authors of the reports with the sense that "the LBV was in an eruptive state at least between 2001 and 2011, which then ended, and may have been followed by a collapse into a massive BH without the production of an SN. This scenario is consistent with the available HST and ground-based photometry."

Or...

A star collapsing into a black hole without a supernova would be a rare event, and that argues against the idea. The paper also notes that we may simply have missed the star's supernova during the eight-year observation gap.

LBVs are known to be highly unstable, so the star dropping to a state of less luminosity or producing a dust cover would be much more in the realm of expected behavior.

Says the paper: "A combination of a slightly reduced luminosity and a thick dusty shell could result in the star being obscured. While the lack of variability between the 2009 and 2019 near-infrared continuum from our X-shooter spectra eliminates the possibility of formation of hot dust (⪆1500 K), mid-infrared observations are necessary to rule out a slowly expanding cooler dust shell."

The authors of the report are pretty confident the star experienced a dramatic eruption after 2011. Beyond that, though:

"Based on our observations and models, we suggest that PHL 293B hosted an LBV with an eruption that ended sometime after 2011. This could have been followed by
(1) a surviving star or
(2) a collapse of the LBV to a BH [black hole] without the production of a bright SN, but possibly with a weak transient."

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