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Chapter 24: Problem 18

Some galaxies in the Local Group exhibit blueshifted spectral lines. Why aren't these blueshifts violations of the Hubble law?

Short Answer

Expert verified
The blueshifts in the Local Group's galaxies do not violate Hubble's law because this law operates on a universal scale, while the blueshifts are local events. These blueshifts are caused by galaxies moving towards us, likely due to gravitational attractions within the Local Group, and do not contradict the overall universe expansion captured by Hubble's law.

Step by step solution

01

Understanding Blueshift and Redshift

Firstly, it is essential to know that the shift in spectral lines, either towards red or blue, signifies the movement of astronomical entities. A blueshift happens when an object is moving towards us, causing the wavelengths of light to decrease and shift towards the blue end of the spectrum. Conversely, a redshift happens when an object is moving away from us, causing the light's wavelengths to increase and shift towards the red end of the spectrum.
02

Comprehending Hubble's Law

Hubble's law posits that objects in the universe are generally moving away from us, the rate of which is directly proportional to their distance from us. Because most galaxies we observe exhibit a redshift, we can infer that they are moving away from us, affirming Hubble's law. This normally presents a paradox since a blueshift signals an object coming towards us.
03

Explaining Local Group Blueshifts

However, these blueshifts in the Local Group do not breach Hubble's law. This is because Hubble's law applies on a universal scale, it provides an overall snapshot of the universe's large-scale structure; it is not as applicable to local scales. In a local group where gravitational interactions between neighboring galaxies play a more substantial role, it is entirely possible and normal for galaxies to move towards each other, causing a blueshift in spectral lines.

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Most popular questions from this chapter

When galaxies pass close to one another, as should happen frequently in a rich cluster, tidal forces between the galaxies can strip away their outlying stars. The result should be a loosely dispersed sea of "intergalactic stars" populating the space between galaxies in a cluster. Search the World Wide Web for information about intergalactic stars. Have they been observed? If so, where are they found? What would our nighttime sky look like if our Sun were an intergalactic star?

Discuss the advantages and disadvantages of using the various standard candle distance indicators to obtain extragalactic distances.

On what grounds do astronomers think that in the past, spiral galaxies were more numerous in rich clusters than they are today? What could account for this excess of spiral galaxies in the past?

The Earth is composed principally of heavy elements, such as silicon, nickel, and iron. Would you be likely to find such planets orbiting stars in the disk of a spiral galaxy? In the nu-cleus of a spiral galaxy? In an elliptical galaxy? In an irregular galaxy? Explain your answers.

Describe what sorts of observations you might make to search for as-yet- undiscovered galaxies in our Local Group. How is it possible that such galaxies might still remain to be discovered? In what part of the sky would these galaxies be located? What sorts of observations might reveal these galaxies?
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