Angular 2 why asterisk (*)

Asterisk syntax is a syntatic sugar for more wordy template syntax which directive expands to under the hood, you are free to use any of these options.

Quote from the docs:

The asterisk is "syntactic sugar". It simplifies ngIf and ngFor for both the writer and the reader. Under the hood, Angular replaces the asterisk version with a more verbose form.

The next two ngIf examples are effectively the same and we may write in either style:

<!-- Examples (A) and (B) are the same -->

<!-- (A) *ngIf paragraph -->
<p *ngIf="condition">
  Our heroes are true!
</p>

<!-- (B) [ngIf] with template -->
<template [ngIf]="condition">
  <p>
    Our heroes are true!
  </p>
</template>

Angular2 offers a special kind of directives - Structural directives

Structural directives are base on the <template> tag.

The * before the attribute selector indicates that a structural directive should be applied instead of a normal attribute directive or property binding. Angular2 internally expands the syntax to an explicit <template> tag.

Since final there is also the <ng-container> element that can be used similarly to the <template> tag but supports the more common short-hand syntax. This is for example required when two structural directives should be applied to a single element, which is not supportd.

<ng-container *ngIf="boolValue">
  <div *ngFor="let x of y"></div>
</ng-container>

Angular treats template elements in a special way. The * syntax is a shortcut that lets you avoid writing the whole <template> element. Let me show you how it works.

using this

*ngFor="let t of todos; let i=index"

translates it into

template="ngFor: let t of todos; let i=index" 

which is then converted into

<template ngFor [ngForOf]="todos" .... ></template>

also Agular's Structural directives like ngFor, ngIf etc. Prefixed by * just to differentiate them from other custom directives and components

see more here