<p>The question <a href="https://astronomy.stackexchange.com/q/35390/7982">Why do they think that WDJ0551+4135 is “snowman shaped” i.e. a contact binary?</a> has also raised the question of what a contact binary is or isn't, and how hard it might be to see one if it is, so let's explore this separately and then hopefully establish some usage guidance for the <a href="/questions/tagged/contact-binaries" class="post-tag" title="show questions tagged &#39;contact-binaries&#39;" rel="tag">contact-binaries</a> tag. </p>

<p><strong>Questions:</strong></p>

<ol>
<li>What's considered "contact" in contact binary white dwarfs? Photosphere contact perhaps? </li>
<li><em>roughly speaking</em>, how long can pairs like this stay in contact before merging? Millenia? Minutes? If the time is very short that means they are much harder to observe in a state of contact.</li>
</ol>

<p>"bonus points:" Are there any contact binaries we can see by eye or with binoculars or a small telescope? (wouldn't have to be white dwarfs) Could we see one vary in brightness and/or color as the pair rotates?</p>

<hr>

<p>From the <a href="https://www.bbc.com/news/science-environment-51707927" rel="noreferrer">BBC article</a> cited in the linked question:</p>

<p><a href="https://i.sstatic.net/clJHr.jpg" rel="noreferrer"><img src="https://i.sstatic.net/clJHrm.jpg" alt="Artwork: The object may be two so-called &#39;white dwarves&#39; merging into a smaller denser star"></a></p>

<blockquote>
  <p>Artwork: The object may be two so-called 'white dwarves' merging into a smaller denser star</p>
</blockquote>

CC BY-SA 4.0
2020-03-03T08:58:42.830Z
@antispinwards Thanks! My thinking is that if "contact" is defined similarly in those cases and the lifetimes were within a few orders of magnitude of each other, it wouldn't really be a broad question. However, it might not be trivial to answer if a reference with a table can't be found.
uhoh
CC BY-SA 4.0
2020-03-03T23:26:35.273Z
@antispinwards okay I've narrowed the scope to white dwarfs.
uhoh
CC BY-SA 4.0
2020-03-04T20:24:26.603Z
Roughly speaking, to merge completely they have to convert a whole lot of orbital angular momentum into the spin of the merged object (or shed it somehow). That's bound to take some time.
PM 2Ring
<white-dwarf><contact-binaries>
What is considered "contact" for contact binary white dwarfs, and roughly how long can they stay that way before merging?
uhoh
# What is considered "contact" for contact binary white dwarfs, and roughly how long can they stay that way before merging?

(Asked by: username_0 on 2020-03-03)

The question [Why do they think that WDJ0551+4135 is “snowman shaped” i.e. a contact binary?](https://astronomy.stackexchange.com/q/35390/7982) has also raised the question of what a contact binary is or isn't, and how hard it might be to see one if it is, so let's explore this separately and then hopefully establish some usage guidance for the [contact-binaries](/questions/tagged/contact-binaries "show questions tagged 'contact-binaries'") tag.

**Questions:**

1. What's considered "contact" in contact binary white dwarfs? Photosphere contact perhaps?
2. *roughly speaking*, how long can pairs like this stay in contact before merging? Millenia? Minutes? If the time is very short that means they are much harder to observe in a state of contact.

"bonus points:" Are there any contact binaries we can see by eye or with binoculars or a small telescope? (wouldn't have to be white dwarfs) Could we see one vary in brightness and/or color as the pair rotates?

---

From the [BBC article](https://www.bbc.com/news/science-environment-51707927) cited in the linked question:

[![Artwork: The object may be two so-called 'white dwarves' merging into a smaller denser star](https://i.sstatic.net/clJHrm.jpg)](https://i.sstatic.net/clJHr.jpg)

> Artwork: The object may be two so-called 'white dwarves' merging into a smaller denser star

**Tags:** white-dwarf, contact-binaries

> **Comments:**
> **username_0**: @antispinwards Thanks! My thinking is that if "contact" is defined similarly in those cases and the lifetimes were within a few orders of magnitude of each other, it wouldn't really be a broad question. However, it might not be trivial to answer if a reference with a table can't be found.  
> **username_0**: @antispinwards okay I've narrowed the scope to white dwarfs.  
> **username_1**: Roughly speaking, to merge completely they have to convert a whole lot of orbital angular momentum into the spin of the merged object (or shed it somehow). That's bound to take some time.  

---

## 0 answers

