How can we be sure that Leavitt’s law works?

[Juju_114]

TL;DR Summary

How can we be certain that our measuring technique (s) are right when measuring celestial objects?

After Henrietta discovered the relationship between luminosity period amongst cephids in the Magellanic Clouds, how was she so sure that you can extend this relationship to other variable stars in other places? This is the part I’m confused because how do we know that these stars are the same to use this principle? Now with other instruments that can allow us to measure further out, have we somehow tested that using leavitt’s law truly works? It seems like we are assuming ALL this Cepheid stars are the same based on their periods. Im sorry if it doesn’t make since, I’m learning

 

[Vanadium 50]

I would not start by calling scientists I did not know by their first names. Some would find that disrespectful.

The behavior of stars is the same thing as the behavior of giant balls of hydrogen and helium. Why would it be different? (c.f. "magic grits", My Cousin Vinny)

Today we also have much better overlap between multiple elements of the distance ladder. We know how far nearby Cepheids are (like Polaris or Delta Cephei) and sure enough, the same relationship. Same argument for ones farther away.

 

[andrew s 1905]

The behavior of stars is the same thing as the behavior of giant balls of hydrogen and helium. Why would it be different? (c.f. "magic grits", My Cousin Vinny)

Because different populations of stars have different constituents in different proportions and are not just balls of hydrogen and helium. Our understanding of stars is not as well defined as you imply.

 

[Vanadium 50]

Our understanding of stars is not as well defined as you imply.

"Our" understanding? I have a MS in Astronomy, What do you understand better than I?

Because different populations of stars have different constituents in different proportions

First, there are two major populations of Cepheids - "classical" (Pop I) and "Pop 2" (obviously, Pop II), sich as W Virginis. This has been known for at least 75 years and is already taken into account.

Second, people have looked at the effect of metallicity on magnitude (or equivalently period) and the effect, if it is even there, is small compared to the spread of individual stars.

Third, the mechanism is understood, and related to the presence of ionized helium. Helium is helium. (Otherwise we are back to 'magic grits;;)

Fourth, Cepheids appear in the "instability strip" on the H-R daigran with other variable stars. They do not appear scattered throughout, as they would if their luminosity were grossly mismeasured.

Finally, now that we have so many overlapping distance scales, we would know if any one scale were problematic.

 

[Juju_114]

I would not start by calling scientists I did not know by their first names. Some would find that disrespectful.

The behavior of stars is the same thing as the behavior of giant balls of hydrogen and helium. Why would it be different? (c.f. "magic grits", My Cousin Vinny)

Today we also have much better overlap between multiple elements of the distance ladder. We know how far nearby Cepheids are (like Polaris or Delta Cephei) and sure enough, the same relationship. Same argument for ones farther away.

You say that we now have some overlap to confirm measurements. I’ve been trying to find sources to read up on. Can you prove them for me?

 

[Juju_114]

You say that we now have some overlap to confirm measurements. I’ve been trying to find sources to read up on. Can you prove them for me?

I have no doubt that these techniques are credible given that scientists find them are, Im just learning more about them and astronomy.

 

[DaveC426913]

You say that we now have some overlap to confirm measurements. I’ve been trying to find sources to read up on. Can you prove them for me?

Not just "some" overlap. The Cosmic Distance Ladder is a big part of the how we get confident about distances. That's the term you're looking for.

 

[DaveC426913]

magic grits

Tee hee.

"Two yoots." "Yoots?" "Yoots."

Now I gotta watch it again.

 

[andrew s 1905]

"Our" understanding? I have a MS in Astronomy, What do you understand better than I?First, there are two major populations of Cepheids - "classical" (Pop I) and "Pop 2" (obviously, Pop II), sich as W Virginis. This has been known for at least 75 years and is already taken into account.

Second, people have looked at the effect of metallicity on magnitude (or equivalently period) and the effect, if it is even there, is small compared to the spread of individual stars.

Third, the mechanism is understood, and related to the presence of ionized helium. Helium is helium. (Otherwise we are back to 'magic grits;;)

Fourth, Cepheids appear in the "instability strip" on the H-R daigran with other variable stars. They do not appear scattered throughout, as they would if their luminosity were grossly mismeasured.

Finally, now that we have so many overlapping distance scales, we would know if any one scale were problematic.

All quite true but not what you originally posted. "Just giant balls of hydrogen and helium".
Not realy the type of helpful input one would expect of a MS in Astronomy. Why not include what you did in your second post.

Your propensity to prefer put downs rather than explanations is sad.

Regards Andrew

While not relevant I have a Phd in physics.

 

[DaveC426913]

Your propensity to prefer put downs rather than explanations is sad.

What??

I don't see any put down but ...

here at PF, we usually let the OP set the tone for how simple or complex the answers are to a question. The OP's opening question is sufficiently broad and basic (which is not a bad thing) that responses are in-kind. If he wanted a more advanced answer, he would have to let us know.

Not realy the type of helpful input one would expect of a MS in Astronomy.

You seem to have put the fault on Vanadium50 for not knowing the OP isn't, say, a Freshman just out of grade school.

 

[Bandersnatch]

*cough* Somebody's having a senior moment *cough*

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Okay, after a Mentor discussion and some PMs, the thread is reopened provisionally. There have been some misunderstandings in this thread, but everybody please be careful to avoid any insults. Thank you.

 

[Ken G]

It depends on if the question being asked is the more technical, "what is the corroborating evidence that supports Leavitt's law", vs. the more general "how do we know stuff in astronomy when we are stuck here on Earth." Assuming the latter, perhaps it would help to start with an understanding that "knowledge" always represents some kind of balancing act between confidence and surprise. When you say, "I just knew that was going to happen," versus "I'm not surprised that happened", you are navigating levels of confidence in ways that are never clearly quantified. It's no different in astronomy, we have things that we have built up a strong sense of confidence about, but no confidence is ever 100 percent, there is always room for surprise. This is not a flaw in science, it is just the way it works.

So when you talk about some specific "law" in astronomy, what you really mean is a demonstrated successful mechanism for making predictions around which you can have confidence. That's it, that's all it means. And the degree of confidence you have depends on the level of accuracy you are shooting for. That's why we talk about "confidence intervals" in science, where we are confident a prediction will be accurate to within some level of uncertainty, but that's all. Our confidence about Leavitt's law is built from a body of corroborating evidence, many of which listed above, which puts us in a position to "win the bet", if you will, about any future observation outside what has already been seen.

However, confidence does not mean we cannot be surprised, we can sometimes "lose our bet." Still, science is a prescription for minimizing that chance. When we send a rocket with people on it to the Moon, we want to have an extremely high confidence we will not lose the bet. But many other times in astronomy, we tolerate less certainty, simply because we have to decide where to invest our resources to learn about things we have much less confidence about. So we will use Leavitt's law to gain leverage over other things we want to understand, given that we have greater confidence in that law than we do the other thing we are studying. Again, that's just the way science makes progress, and people get to the Moon, and diseases get cured, and so on. It's never a map to certainty.

 

[Vanadium 50]

I'm not sure the OP cares any more, but its the properties of helium - specifically, its ionization - that drives the oscillation.

To oversimplify, you have a phase transition between singly and doubly ionized helium, and this changes the thermodynamic properties of the H-He mixture above and below the transition. This drives the pulsation.

Insofar as the composition of stars is more or less the same everywhere, this "instability strip" which contains Cepheids is the same everywhere.

 

[Ken G]

And to expound on that a little more, if the OP does indeed still care, the issue is where in the star is located the temperature regime that @Vanadium 50 is talking about, where the change in ionization occurs. For cooler stars, high enough temperatures are achieved too deep in the star to be able to drive the oscillation, and for hotter stars, it happens too far out, or not all. So that's why there is a "strip" in the surface temperatures of stars where the instability is able to make the star pulsate, and the smaller the star, the faster it can be made to pulsate. A smaller star that's closer looks a lot like a larger star that's farther away, but if it pulsates faster, it must be the smaller, closer version, not the bigger, farther away version. So yes, there is a good reason to expect Leavitt's law to work, and it has passed tests, so it's not purely an assumption that it will work somewhere it is being applied. But of course, we always keep an eye out for surprises.

 

[Vanadium 50]

As a gee-whiz fact, if the sun were a Cepheid, it would have a period of a few minutes. And the sun does have an oscillation of a few minutes.

However, these are unrelated. The sun's variability on this timescale is driven by surface effects, it may well have a few minute pulsation deep below the surface, but the long time scale to transport the energy to the surface washes it out.

As Ken G alludes to, the physics that drives the pulsation is always there. But for some stars it happens way out in the atmosphere, where it doesn't matter, and for others it happens deep in the star where it doesn't matter either. A Cepheid has it happening near the surface, and the effect is to expand the surface without substantially cooling it, causing the star to brighten. (And the reverse in the other half of the cycle)

There are also stars called 'dwarf Cepheids", a term I dislike. I prefer Delta Scuti variables. More straightforward. I know some people use them as standard candles, but they would not be my first choice. First, they're smaller and thus dimmer than classical Cepheids. So there are usually other options. Also, they are fast rottaors, and thus oblong, and equatorial and polar pulsations are different. People use them, but they wouldn't be my first choice.